Peptides derived from prosomatostatins I and II and from two distinct proglucagons have been isolated from the pancreas of a teleost fish, the European eel ( Anguilla anguilla). The product of prosomatostatin I processing, somatostatin-14, is identical to mammalian somatostatin-14. A 25-amino-acid-residue peptide (Ser-Val-Asp-Asn-Gln 5-Gln- Gly-Arg-Gly-Arg 10-Lys-Ala-Gly-Cys-Lys 15-Asn-Phe-Tyr-Trp-Lys 20-Gly-Pro- Thr-Ser-Cys 25) is derived from prosomatostatin II. Compared with the corresponding peptides from other teleost fish, the eel somatostatin-25 contains the unusual substitution Pro for Phe at position 22. This peptide was also isolated in a form containing a hydroxylysyl residue at position 20. A 29-amino-acid-residue eel glucagon contains four substitutions relative to human glucagon Asn for Ser 8, Glu for Asp 15, Thr for Ser 16, and Ser for Thr 29). In common with mammalian and avian glucagons but unlike most other fish glucagons, the eel peptide possesses a glutamine residue at position 3. A peptide derived from a second proglucagon comprises 36 amino acid residues. A 7-residue C-terminal extension to the glucagon sequence shows structural similarity to the corresponding extension in ratfish ( Hydrolagus colliei) glucagon and mammalian oxyntomodulin.