Anglerfish (Lophius americanus) insulin complementary DNA was cloned in bacterial plasmids, and its sequence was determined. Fish insulin messenger RNA is larger (1.5 times) than the messenger RNA encoding mammalian (rat and human) insulin, in part because of a larger C peptide (an additional six amino acids or 18 nucleotides in length) but mainly because of increases in the 5' and 3' untranslated regions. Comparison of the fish, rat, and human insulin messenger RNA (from the complementary DNA) reveals that, in addition to the regions coding for the A and B peptides, sequence conservation is limited to a segment within the 5' untranslated region which may be involved in ribosomal binding, two small segments of the signal peptide, and two stretches of sequence in the 3' untranslated region.