It has been regarded that Vibrio cholerae O1 inhabit in environmental water. As many cholera patients emerge in Kolkata, it has been thought that V. cholerae O1 is easily detected in environmental water in Kolkata. However, the detection of V. cholerae O1 is rare, though other V. cholerae (NAG Vibrio) is constantly detected. To clear the reason for the difference of the detection rate of two Vibrios, we examined the viability of V. cholera O1 and NAG Vibrios in low ionic strength aquatic medium. We observed greater declining viability of V. cholerae O1 possessing cholera toxin gene (ctx) in low ionic strength solution, but the decline of NAG Vibrios non-possessing ctx is small. To evaluate the concerning of ctx in the viability, we examined the viabilities of V. cholerae O1which do not possess ctx and NAG Vibrios possessing ctx under the same condition. The result indicated that the existence of the ctx induces the decrease the viability of the host in low ionic strength solution. The decrease observed in this experiment might relate with the low detection of V. cholerae O1 possessing ctx in environmental water, though NAG Vibrio is constantly detected.