Three newly isolated strains of flavobacteria from coastal aquifer sediments have been found to be predatory, lysing a range of live and pasteurized microbial prey. The three strains have been classified on the basis of 16S rRNA gene phylogeny as belonging to the recently described Olleya (strains VCSA23 and VCSM12) and Tenacibaculum (strain VCSA14A) genera. Two of the closest cultured relatives to the strain VCSA14A, Tenacibaculum discolor and Tenacibaculum gallaicum, were also found to be bacteriolytic. These five predatory strains exhibit gliding motility and have been observed to lyse prey cells after surrounding them with social swarms, similar to known predatory bacteria such as myxobacteria and members of the genus Lysobacter. Flavobacteria are often numerically significant in a wide variety of freshwater and marine environments, particularly in association with particles, and are thought to be involved in the degradation of biopolymeric substances. If predatory capability is widespread among flavobacteria, they may be a previously unrecognized source of 'top-down' bacterial mortality with an influence on the composition and activity of surrounding microbial communities.