Aquatic Coleoptera assemblages present in floating vegetation in the ecotone between subtropical forest and pampasic grassland in Punta Lara (Buenos Aires, Argentina) were characterized and compared. Four pools, one located in the forest, two in the grassland and one in the forest/grassland ecotone, were sampled during a two years period (July 1996–June 1998). For each sampling station species richness and diversity were calculated, and they were compared through two similarity coefficients (Jaccard and Winer). Aquatic species of Dryopidae, Hydrochidae, Hydrophilidae, Hydraenidae, Limnichidae, Scirtidae, Haliplidae, Gyrinidae, Dytiscidae and Noteridae were identified. Highest species richness was registered in the grasslands, and lowest in the forest/grassland ecotone. Cluster analysis showed highest similarity between grassland areas, and between these and the intermediate pool, and lowest between the forest and the other areas. Biogeographical remarks are discussed.

Publication funded by the Patricia Vaurie bequest.