A salmon cystatin cDNA clone was isolated from a chum salmon cDNA library. The clone encoded a full-length extracellular-type cystatin and its signal peptide and included 5'- and 3'-untranslated regions. The deduced amino acid sequence showed a high degree of sequence similarity to mammalian cystatin C, chicken egg cystatin, and chum salmon pituitary cystatin. By Northern blot analysis, the salmon cystatin was found to show apparently non-tissue specific expression. Because platyfish EHS cells transfected with a cystatin expression vector produced a 13 kDa mature cystatin in the culture medium, the salmon cystatin was considered to act as an extracellular type of cystatin in the fish cells. These findings indicate that the salmon cystatin is a homolog of mammalian cystatin C.