Previously we have shown that the open reading frame 2 protein (ORF2 protein), which is encoded at the 3 ' end of serine protease of Aeromonas sobria (ASP), functions as a chaperone protein in periplasm in the production of ASP. Both proteins, ASP and ORF2 protein, associate in periplasm and ORF2 protein helps ASP to take an active form. ASP which is dissociated from ORF2 protein emerges in milieu . In this study, we examined the effect of sodium chloride (NaCl) in medium on ASP production by A. sobria. The ASP activity of culture supernatant was extremely decreased when A. sobria was cultured in medium containing 3.0% NaCl (concentration almost equivalent to sea water salinity). Our analysis showed that the transcription of asp by A. sobria is not inhibited by NaCl in medium and that A. sobria synthesizes and releases ASP in milieu even under the condition of 3.0% NaCl. However, these ASPs in milieu formed complex as with ORF2 proteins. This indicates that the maturation pathway of ASP is disturbed in A. sobria cultured in medium containing 3.0% NaCl. It is likely that ASP does not associate with ORF2 protein in the correct form in periplasam when A. sobria is cultured in medium containing 3.0% NaCl, though both proteins, ASP and ORF2 protein, make complexes and emerge outside of the cell. This idea suggests that the chaperone system of ASP possesses the ability to sense NaCl in surroundings and regulates the production of active ASP.