Exhibit 5.1

Cahill Gordon & Reindel llp
32 Old Slip
New York, NY 10005

HELENE R. BANKS ANIRUDH BANSAL DAVID L. BARASH LANDIS C. BEST BRADLEY J. BONDI BROCKTON B. BOSSON JONATHAN BROWNSON * JOYDEEP CHOUDHURI * JAMES J. CLARK CHRISTOPHER W. CLEMENT LISA COLLIER AYANO K. CREED PRUE CRIDDLE ± SEAN M. DAVIS STUART G. DOWNING ADAM M. DWORKIN ANASTASIA EFIMOVA JENNIFER B. EZRING HELENA S. FRANCESCHI JOAN MURTAGH FRANKEL JONATHAN J. FRANKEL ARIEL GOLDMAN PATRICK GORDON JASON M. HALL STEPHEN HARPER WILLIAM M. HARTNETT NOLA B. HELLER CRAIG M. HOROWITZ DOUGLAS S. HOROWITZ TIMOTHY B. HOWELL DAVID G. JANUSZEWSKI ELAI KATZ JAKE KEAVENY BRIAN S. KELLEHER RICHARD KELLY CHÉRIE R. KISER ‡ JOEL KURTZBERG TED B. LACEY MARC R. LASHBROOK ALIZA R. LEVINE JOEL H. LEVITIN GEOFFREY E. LIEBMANN TELEPHONE:  (212) 701-3000
WWW.CAHILL.COM
___________

1990 K STREET, N.W.
WASHINGTON, DC 20006-1181
(202) 862-8900

CAHILL GORDON & REINDEL (UK) LLP
24 MONUMENT STREET
LONDON EC3R 8AJ
+44 (0) 20 7920 9800
___________

WRITER’S DIRECT NUMBER BRIAN T. MARKLEY MEGHAN N. McDERMOTT WILLIAM J. MILLER NOAH B. NEWITZ WARREN NEWTON § DAVID R. OWEN JOHN PAPACHRISTOS LUIS R. PENALVER KIMBERLY PETILLO-DÉCOSSARD SHEILA C. RAMESH MICHAEL W. REDDY OLEG REZZY THORN ROSENTHAL TAMMY L. ROY JONATHAN A. SCHAFFZIN DARREN SILVER JOSIAH M. SLOTNICK RICHARD A. STIEGLITZ JR. ROSS E. STURMAN SUSANNA M. SUH ANTHONY K. TAMA JONATHAN D. THIER SEAN P. TONOLLI JOHN A. TRIPODORO GLENN J. WALDRIP, JR. HERBERT S. WASHER MICHAEL B. WEISS DAVID WISHENGRAD C. ANTHONY WOLFE COREY WRIGHT ELIZABETH M. YAHL JOSHUA M. ZELIG      * ADMITTED AS A SOLICITOR IN
ENGLAND AND WALES ONLY  ± ADMITTED AS A SOLICITOR IN
WESTERN AUSTRALIA ONLY  ‡ ADMITTED IN DC ONLY  § ADMITTED AS AN ATTORNEY
IN THE REPUBLIC OF SOUTH AFRICA
ONLY

August 12, 2021

Dyadic International, Inc.

140 Intracoastal Pointe Drive, Suite 404

Jupiter, Florida 33477

Re:         Dyadic International, Inc.

Ladies and Gentlemen:

We have acted as counsel to Dyadic International, Inc., a Delaware corporation (the “Company”), in connection with the Post-Effective Amendment No. 1 to the Registration Statement on Form S-8 (File No. 333-231712) (the “Registration Statement”) filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), with respect to 6,712,601 shares of the Company’s Common Stock, par value $0.001 per share (the “Common Stock”) previously registered for issuance under the Dyadic International, Inc. 2011 Equity Incentive Plan (the “2011 Plan”), consisting of (a) 1,388,386 shares of Common Stock previously authorized under the 2011 Plan, which, upon shareholder approval of the Dyadic International, Inc. 2021 Equity Incentive Plan (the “2021 Plan”), became available for issuance under the 2021 Plan, and (b) 5,324,215 shares of Common Stock previously authorized and subject to outstanding awards under the 2011 Plan, which may become available for issuance under the 2021 Plan, provided the awards are forfeited or lapse unexercised (the shares described in (a) and (b), the “2011 Plan Shares”).

We have examined copies of such corporate records, instruments and certificates and made such inquiries as we have deemed necessary in connection with the opinion set forth below. In our examination, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals and the conformity to originals of all documents submitted to us as copies.

Cahill Gordon & Reindel llp

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Based upon the foregoing, in our opinion the 2011 Plan Shares to be issued by the Company, when issued in the manner contemplated by the 2021 Plan, will be legally issued, fully paid and non-assessable. We express no opinion as to the laws of any jurisdiction other than the General Corporation Law of the State of Delaware and the federal laws of the United States of America.

We hereby consent to the filing of a copy of this opinion with the Commission as an exhibit to the Registration Statement. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission thereunder.

Best regards,

/s/ Cahill Gordon & Reindel llp

Cahill Gordon & Reindel llp