Co-existence of trh gene and urea-hydrolysing property in one of 44 marine water isolates of Vibrio parahaemolyticus correlates strongly with both genotypic and phenotypic characteristics of the bacterium. Thus, urease-producing phenotype can be considered a marker of virulence for the production of thermostable direct haemolysin-related haemolysin (TRH) (i.e. possession of trh gene). The same isolate also possessed the tdh gene. An environmental isolate possessing all the characteristics of a pathogenic V. parahaemolyticus in this marine environment suggest that there is a likelihood of the occurrence of clinical cases of gastroenteritis caused by V. parahaemolyticus in the Andamans. © 1998 International Centre for Diarrhoea! Disease Research,.