We have cloned two distinct cDNAs encoding salmon-type calcitonin (sCT)-I cDNAs from the ultimobranchial gland of rainbow trout, Oncorhynchus mykiss. Both cDNAs were predicted to encode nearly identical sCT-I precursors which consisted of an N-terminal peptide of 80 amino acid residues, a putative cleavage site Lys-Arg, sCT-I, a cleavage and amidation sequence Gly-Lys-Lys-Arg, and a C-terminal peptide of 18 amino acids. Development of sCT-I-expressing cells was then examined by employing conventional histochemical staining, in situ hybridization with a specific cRNA probe, and further immunohistochemistry. The primordium of the ultimobranchial gland was first identified, as two cell masses, in the region between the alimentary canal and sinus venosus behind the heart 17 days postfertilization (dpf; 14°C). However, expression of sCT-I mRNA could not be detected in this gland until one day later, and appeared at 18 dpf. sCT-I immunoreactivity was first observed at 19 dpf (two days before hatching), and the ultimobranchial gland began to assume a follicular structure at 20 dpf (one days before hatching). As ontogeny proceeded, the sCT-I-immunoreactive cells increased in both number and stainability. The sCTI mRNA was also expressed on the developing gill filaments, but immunoreactive sCT-I was not detected in these sites. These results provide basic data for further research on the organogenesis of the trout ultimobranchial gland.