Glucagon has been isolated from the endocrine pancreas of a tunid, Thunnus obesus. The primary structure of the glucagon molecule was established as H S E G T F S N D Y S K Y L E T R R A Q D F V Q W L K N S. The sequence is identical to those of sculpin and flounder glucagon and glucagon II from anglerfish. It also shows high homology to the mammalian hormone (76%). The mass determined by fast-atom bombardment (3508) was consistent with the proposed structure. Immunological properties of the tuna glucagon were analyzed by radioimmunoassay, showing a high degree of cross-reactivity with the 30K antibody.