The activity of membrane-bound NADH oxidase of Vibrio anguillarum M93 (serotype J-O-1), which causes vibriosis in freshwater area was activated by Na(+) in the same manner as other marine Vibrios. However, in addition to Na(+), K(+) was also found to positively enhance the NADH oxidase activity of strain M93. This tendency has not been recognized in other marine Vibrios. Furthermore, the Na(+)-dependent NADH oxidase of strain M93 required less Na(+) (0.1 M) for its maximum activity than those of other Vibrios such as Vibrio alginolyticus and 'Vibrio angustum' S14, which were in the range of 0.4 M NaCl, similar as seawater. Destruction of H(+) motive force by a proton conductor carbonylcyanide m-chlorophenylhydrazone (CCCP) revealed high dependency of V. anguillarum on the primary H(+) pump. Even at pH 8.5, V. anguillarum strains other than serotype O-4 could not grow well with the addition of CCCP. In contrast, marine-type Vibrios such as V. alginolyticus and V. angustum S14 can grow well at pH 8.5 even with the addition of CCCP. The lower requirement for Na(+) in V. anguillarum probably reflects the salinity of their original habitats.