Classical and El Tor strains of Vibrio cholerae were examined for production of bacteriophage or bacteriophage-related material by electron microscopy of mitomycin C-induced cultures. The strains were also tested for pathogenicity by using the ligated ileal loops of adult rabbits. Of the 27 strains tested, 25 showed a correlation between the production of bacteriophage-related material and pathogenicity. Both P(+) and P(-) strains produced bacteriophage tails. The bacteriophage tails of strains 569B and ATCC 9168 and other strains were shown to occur in an extended form with tail appendages.