A novel calcitonin (CT) was isolated from the spotlined sardine, Sardinops melanostictus. The primary structure of sardine CT was determined as follows: H-Cys-Ser-Asn-Leu-Ser-Thr- Cys-Ala-Leu-Gly-Lys-Leu-Ser-Gln-Glu-Leu-His-Lys-Leu-Gln-Ser-Tyr-Pro-Arg- Thr- Asn-Val-Gly-Ala-Gly-Thr-Pro-NH2. This amino acid sequence was different from that of salmon CT in 4 amino acid residues at positions 8th, 21th, 27th and 29th. As judged by the international method of CT bioassay, hypocalcemic activity of sardine CT was calculated as 4156 IU/mg. When compared for durability of CTs, it was found that sardine CT was significantly more potent than that of salmon CT. This is the first report of CT from a marine species of teleost.