Eighteen bacteriophages lytic for Vibrio parahaemolyticus were isolated from seawater, and their ultrastructure was examined by electron microscopy. Based on the phage morphology, they were classified in four groups. Group I phages consisted of a hexagonal head and a tail with a contractile sheath. All the phages of the other three groups had a relatively long, noncontractile tail, but there were differences in the head structure among these phages. The phages of groups II and III had a hexagonal head and an elongated polyhedral head, respectively. Group IV phages exhibited a unique hexagonal head with knoblike projections. There appeared to be no correlation between the O and K serotypes of V. parahaemolyticus strains and the host ranges of the phages. The phages had varying sensitivities to heat and organic solvents.