The abundance and structure of heterotrophic bacterial populations in an eelgrass Zostera marina field were investigated. For all the samples, the density of the aerobic bacteria was approximately tenfold higher than that of the anaerobic bacteria. Distinct differences were found between the composition of the bacterial population in rhizosphere, surrounding sediment and bottom seawater both with the aerobic and the anaerobic cultivations. The aerobic bacterial population of the bottom seawater and sediment was composed mainly of Acinetobacter-Moraxella and Flavobacrerium, whereas in eelgrass rhizosphere pseudomonads and halophilic aeromonads as well as Acinetobacter-Moraxella were dominant. Aeromonas spp. were also dominant among the anaerobic bacteria in eelgrass rhizosphere, in contrast with Vibrio spp. in the surrounding sedimentand in the bottom seawater. While the aerobic bacteria rarely showed antibacterial activity, the anaerobic bacteria, especially Aeromonas spp. from the rhizosphere, exhibited a high percentage of antibacterial activity against Staphylococcus aureus.