Oligonucleotide probes based on the known amino acid sequence of salmon calcitonin were used to screen a cDNA library obtained from ultimobranchial glands of salmon for clones encoding salmon calcitonin. From the cDNA sequence of strongly hybridizing clones the complete primary structure of the calcitonin precursor could be deduced. Its overall structure is identical with the structures of procalcitonins from other vertebrates and has the highest homology with the chicken precursor.