Eighty-one microbial species isolated from seafoods and other marine sources were examined to determine the extent of interactions among these species. Spot-plates, cross-plates, and concurrent growth experiments at 7, 15, 20, and 30 C indicated that Lactobacillus species were capable of inhibiting other microorganisms. Lactobacillus species isolated from oysters and identified as Lactobacillus plantarum produced a substance inhibitory to Pseudomonas, Bacillus, and Proteus species, the most sensitive being Pseudomonas. The inhibitory substance accumulated in Lactobacillus culture media, reaching maximum concentrations in 4 to 5 days at 30 C. The active substance was dialyzable, heat labile, and inactivated by catalase. Inhibitor production paralleled H2O2 formation in Lactobacillus cultures, further indicating that the observed inhibition resulted from H2O2 produced by lactobacilli. These findings may explain the abnormal shifts in microbial flora observed in foods where Lactobacillus species have overgrown the natural flora.