Abstract Observations of a spontaneous infection in Crassostrea virginica larvae at Milford Laboratory indicate that the causative microorganism, Vibrio sp., was present in the filtered, uv light-treated sea water in very low concentrations. The disease was usually noted within 6–10 days after fertilization of eggs and mortality was usually complete about 6–8 days later. The disease could be induced by adding either washed cells of the pathogen or the broth filtrate. Observations made when the filtrate was employed indicate that the bacterium produced a toxin capable of breaking down the tissue structure of the oyster larvae. Mortalities were prevented with 100 mg of neomycin/liter of oyster egg culture.