Bacteria in the genus Vibrio can be opportunistic pathogens for organisms in marine and estuarine systems. Consumption of seagrass by the blue crab Callinectes sapidus or macroalgal kelp by the lobster Homarus americanus reduced epibiotic bacteria in the Vibrio genus. Bacterial Vibrio spp. densities were estimated by colony growth on thiosulfate citrate bile sucrose agar. Nonspecific epibiotic bacterial densities were estimated by colony growth on nonselective agar or by direct counting under a fluorescence microscope. Consumption of the seagrass Ruppia maritima, but not the aquatic plant Stukenia pectinata or a control diet, reduced Vibrio spp. densities on blue crabs. A diet containing the kelp Saccharina latissima reduced Vibrio spp. densities on lobsters compared with a control diet. Results for nonspecific bacterial densities were mixed. A reduction was observed when cultured colonies were counted but not when cells were counted directly. The results suggest that diets containing seagrass or kelp can reduce epibiotic bacterial densities of some but not all bacterial species. This reduction may involve the utilization of antibacterial compounds obtained through diet and could enhance animal health by inhibiting growth of potential pathogens.