The presence of piscivorous fish has a significant effect on the structure of fishassemblages. Exotic, piscivorous largemouth bass (Micropterus salmoides)have been introduced to a number of Japanese farm ponds that were originally characterizedby a high level of biodiversity. We evaluated the effects of exotic bass on the structureof the fish assemblage following their introduction into the farm ponds. We collected fishand measured environmental conditions in 14 ponds of northeastern Japan. The speciesrichness of pond-dwelling fish assemblages was negatively correlated with the relativeabundance of exotic bass. In addition, the relative abundance of lentic cyprinid species(e.g., Rhodeus ocellatus ocellatus, Pseudorasbora parvaand Carassius sp.) was negatively correlated with the relativeabundance of exotic bass. The abundance of these species was also influenced byenvironmental factors, including transparency and/or coverage of aquatic vegetation,likely related to their influence on the predatory efficiency of exotic bass. Our resultssuggest that the structure of fish assemblages in farm ponds with exotic piscivorous fishwas influenced by both biotic (i.e., predation) and abiotic factors and that someenvironmental factors mediate the effects of exotic bass on fish species richness andcomposition.