Vibrio species have been suggested as indicators of water quality. Methods for the isolation, identification, and enumeration of V. cholerae and V. parahaemolyticus (Biotype 1, parahaemolyticus, and Biotype 2, alginolyticus) have been developed and successfully applied in analyses of water. Recent isolations of V. cholerae from a harbor in the United States suggest that greater attention should be paid to the Vibrio species as waterborne pathogens. The identification and classification of V. cholerae and the two biotypes of V. parahaemolyticus have been greatly advanced. Other related Vibrio species remain to be described, and practical methods for their enumeration are not yet available. Because Vibrio species occur throughout the world in bays, estuaries, and marine waters, it is imperative that the ecology of this bacterial group be elucidated.