Castle Biosciences, Inc. (CSTL) Ownership
CSTL ownership summary
as of October 18, 2026- Institutions
- 90.9% of shares outstanding
- Insiders
- 2.38% of shares outstanding
- Shares outstanding
- 30.5M
- Float
- 27.2M shares available to trade
- Short interest
- 6.70% 2.0M shares short
- Days to cover
- 5.82 short interest up 5.3% vs prior month
- Institutions 90.89%
- Insiders 2.38%
- Other investors 6.73%
CSTL top institutional holders
20 largest| # | Holder | % of shares | Shares | Value | Change | Reported |
|---|---|---|---|---|---|---|
| 1 | BlackRock Inc | 10.78% | 3.3M | $105.2M | +309,017 (+10.4%) | June 30, 2026 |
| 2 | RTW INVESTMENTS, LLC | 9.09% | 2.8M | $88.8M | +20,000 (+0.7%) | June 30, 2026 |
| 3 | Principal Financial Group Inc | 5.65% | 1.7M | $55.2M | −43,229 (−2.4%) | June 30, 2026 |
| 4 | Vanguard Capital Management, LLC | 4.01% | 1.2M | $39.2M | +48,975 (+4.2%) | June 30, 2026 |
| 5 | Acadian Asset Management LLC | 3.01% | 919K | $29.4M | +434,227 (+89.6%) | June 30, 2026 |
| 6 | Allianz Asset Management AG | 2.88% | 879K | $28.1M | +67,934 (+8.4%) | June 30, 2026 |
| 7 | Vanguard Portfolio Management, LLC | 2.75% | 841K | $26.9M | −20,986 (−2.4%) | June 30, 2026 |
| 8 | Summit Partners Public Asset Management, LLC | 2.70% | 825K | $26.4M | −115,813 (−12.3%) | June 30, 2026 |
| 9 | State Street Corp | 2.69% | 822K | $26.3M | +53,000 (+6.9%) | June 30, 2026 |
| 10 | Geode Capital Management, LLC | 2.61% | 796K | $25.4M | +67,812 (+9.3%) | June 30, 2026 |
| 11 | Mackenzie Investments | 2.60% | 794K | $25.4M | +719,396 (+970.7%) | June 30, 2026 |
| 12 | Dimensional Fund Advisors, Inc. | 2.59% | 791K | $25.3M | −248,676 (−23.9%) | June 30, 2026 |
| 13 | Jefferies Financial Group Inc | 2.12% | 647K | $20.7M | +647,391 (0.0%) | June 30, 2026 |
| 14 | Qube Research & Technologies | 1.93% | 589K | $18.8M | +29,717 (+5.3%) | June 30, 2026 |
| 15 | AIGH Capital Management, LLC | 1.78% | 545K | $17.4M | −45,500 (−7.7%) | June 30, 2026 |
| 16 | Renaissance Technologies Corp | 1.22% | 371K | $11.9M | −161,600 (−30.3%) | June 30, 2026 |
| 17 | The Goldman Sachs Group Inc | 1.17% | 358K | $11.5M | +17,226 (+5.0%) | June 30, 2026 |
| 18 | Point72 Asset Management, L.P. | 1.17% | 357K | $11.4M | +357,317 (0.0%) | June 30, 2026 |
| 19 | Kennedy Capital Management Inc | 1.14% | 348K | $11.1M | +348,371 (0.0%) | June 30, 2026 |
| 20 | Connor Clark & Lunn Inv Mgmt Ltd | 1.09% | 332K | $10.6M | −45,355 (−12.0%) | June 30, 2026 |
Value at the latest close. Change = shares bought or sold in the latest quarterly filing (13F). In the top 20, 13 holders added 3.1M shares and 7 cut 681K.
CSTL top mutual fund and ETF holders
20 largest| # | Holder | % of shares | Shares | Value | Change | Reported |
|---|---|---|---|---|---|---|
| 1 | Vanguard MStar Total Stk Mkt Idx Invest | 2.79% | 853K | $27.3M | — | August 31, 2026 |
| 2 | Strategic Advisers U.S. Total Stock | 2.60% | 793K | $25.4M | +306,743 (+63.0%) | August 31, 2026 |
| 3 | iShares Russell 2000 ETF | 2.54% | 776K | $24.8M | −8,960 (−1.1%) | September 30, 2026 |
| 4 | Principal SmallCap R5 | 1.94% | 591K | $18.9M | −133,831 (−18.5%) | August 31, 2026 |
| 5 | Canada Life US Small-Mid Cap Growth F | 1.11% | 340K | $10.9M | −34,470 (−9.2%) | July 31, 2026 |
| 6 | Fidelity Small Cap Index | 1.07% | 328K | $10.5M | +3,261 (+1.0%) | July 31, 2026 |
| 7 | iShares Russell 2000 Value ETF | 0.92% | 280K | $9.0M | +426 (+0.2%) | September 30, 2026 |
| 8 | Wasatch Micro Cap | 0.91% | 277K | $8.8M | — | June 30, 2026 |
| 9 | Vanguard Explorer Inv | 0.90% | 276K | $8.8M | — | June 30, 2026 |
| 10 | Buffalo Small Cap Growth | 0.78% | 240K | $7.7M | — | June 30, 2026 |
| 11 | iShares Biotechnology ETF | 0.77% | 234K | $7.5M | −1,776 (−0.8%) | September 30, 2026 |
| 12 | BlackRock Advantage Small Cap Core Instl | 0.62% | 190K | $6.1M | — | August 31, 2026 |
| 13 | Vanguard Strategic Small-Cap Equity Inv | 0.57% | 175K | $5.6M | +6,573 (+3.9%) | June 30, 2026 |
| 14 | Vanguard Russell 2000 ETF | 0.56% | 172K | $5.5M | +3,974 (+2.4%) | August 31, 2026 |
| 15 | Schwab US Small-Cap ETF™ | 0.53% | 162K | $5.2M | +108 (+0.1%) | September 30, 2026 |
| 16 | Fidelity Extended Market Index | 0.49% | 150K | $4.8M | +28,500 (+23.5%) | August 31, 2026 |
| 17 | EA Bridgeway Omni Small-Cap Value ETF | 0.49% | 149K | $4.8M | — | September 30, 2026 |
| 18 | DFA US Small Cap I | 0.44% | 135K | $4.3M | — | August 31, 2026 |
| 19 | Advisors Capital Small/Mid Cap | 0.44% | 135K | $4.3M | — | June 30, 2026 |
| 20 | Allspring Emerging Growth Inst | 0.44% | 134K | $4.3M | — | August 31, 2026 |
Funds report holdings monthly or quarterly; dates differ by fund.
CSTL insider transactions
latest 20| Date | Name | Transaction | Price | Shares | Value |
|---|---|---|---|---|---|
| 2026-04-21 | Derek J Maetzold | Sell | $25.10 | 3,622 | $90,912 |
| 2026-04-21 | Derek J Maetzold | Sell | $25.10 | 550 | $13,805 |
| 2026-04-06 | Derek J Maetzold | Sell | $25.03 | 6,214 | $155,536 |
| 2026-04-06 | Derek J Maetzold | Sell | $25.03 | 3,622 | $90,659 |
| 2026-03-18 | Derek J Maetzold | Sell | $25.39 | 650 | $16,504 |
| 2026-03-17 | Derek J Maetzold | Sell | $25.93 | 18,650 | $483,594 |
| 2026-03-12 | Tobin W Juvenal | Sell | $25.24 | 20,863 | $526,582 |
| 2026-03-05 | Frank Stokes | Sell | $27.98 | 6,001 | $167,908 |
| 2026-03-05 | G Bradley Cole | Sell | $28.47 | 7,403 | $210,763 |
| 2026-03-03 | Frank Stokes | Sell | $27.44 | 7,000 | $192,080 |
| 2026-02-04 | Derek J Maetzold | Sell | $37.99 | 817 | $31,038 |
| 2026-02-03 | Derek J Maetzold | Sell | $39.91 | 3,200 | $127,712 |
| 2026-01-20 | Derek J Maetzold | Sell | $40.01 | 4,017 | $160,720 |
| 2026-01-06 | Derek J Maetzold | Sell | $40.10 | 1,339 | $53,694 |
| 2026-01-05 | Derek J Maetzold | Sell | $38.09 | 2,678 | $102,005 |
| 2025-12-22 | Derek J Maetzold | Sell | $40.35 | 830 | $33,490 |
| 2025-12-17 | Derek J Maetzold | Sell | $40.04 | 509 | $20,380 |
| 2025-12-17 | Kristen M Oelschlager | Sell | $39.31 | 14,888 | $585,247 |
| 2025-12-04 | Derek J Maetzold | Sell | $40.03 | 1,339 | $53,600 |
| 2025-11-25 | Derek J Maetzold | Sell | $40.05 | 5,115 | $204,856 |
Reported trades by company officers, directors and other filers (including disclosures by members of Congress where EODHD lists them). "—" where the filing gives no share count.
CSTL shares outstanding
Year-end shares outstanding, adjusted for splits.
Who owns CSTL
Institutional investors own 90.9% of Castle Biosciences's shares and insiders 2.38%. The largest holder is BlackRock Inc with 10.78% of the shares (3.3M), as of June 30, 2026. Among the top 20 institutions, 13 added shares and 7 reduced their stake in the latest filings. Short interest is 6.70% of the float, or 5.82 days of average trading volume. The share count rose 175.4% since 2018.
Related: CSTL analyst forecast, CSTL key metrics. See the CSTL stock overview for the latest quote and news.
CSTL ownership FAQ
Who owns the most Castle Biosciences stock?
The largest shareholders of Castle Biosciences are BlackRock Inc (10.78%), RTW INVESTMENTS, LLC (9.09%), Principal Financial Group Inc (5.65%), Vanguard Capital Management, LLC (4.01%), Acadian Asset Management LLC (3.01%).
What percentage of Castle Biosciences is owned by institutions?
Institutions hold 90.9% of Castle Biosciences's shares outstanding; insiders hold 2.38%.
Which funds own Castle Biosciences?
The largest fund holders of Castle Biosciences are Vanguard MStar Total Stk Mkt Idx Invest (2.79%), Strategic Advisers U.S. Total Stock (2.60%), iShares Russell 2000 ETF (2.54%), Principal SmallCap R5 (1.94%), Canada Life US Small-Mid Cap Growth F (1.11%).
What is Castle Biosciences's short interest?
2.0M shares of Castle Biosciences were sold short, 6.70% of the float; at the average daily volume it would take 5.82 days to cover.
Are insiders buying or selling Castle Biosciences?
Of the latest 20 reported insider transactions in Castle Biosciences (since November 25, 2025), 0 were purchases and 20 were sales.
How many shares of Castle Biosciences are outstanding?
Castle Biosciences has 30.5M shares outstanding, of which 27.2M are in the public float.