The bactericidal effect of water from the Indian and Pacific oceans and Caspian and White seas on Escherichia coli was studied. It is shown that a decrease in the viability of E. coli cells is accompanied by the appearance and active multiplication of small bacteria of a vibrioid form. Two strains of such bacteria were isolated from a suspension of E. coli in seawater and purified from membranes of the substrate organism by the method of centrifugation in a Ficoll gradient. The marine bacteria of vibrioid form are very similar to the bacterial parasite Bdellovibrio bacteriovorus in fatty acid composition and characteristic cycle of development.