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Benthic fauna from lakes and streams of the Iberá system (Corrientes, Argentina) The taxonomic composition , diversity and abundance of bottom dwelling invertebrates are described for a series of lakes and streams located in an area peculiar for its extensive development of swamps. A preliminary classification of these waterbodies using invertebrates, is proposed. The lakes are surrounded and endroached upon by swampy vegetation forming 'embalsados' ('sudss') which can reduce substantially the open water surface. Submerged vegetation is also often abundant. The lakes are shallow (<3.5 m) and are characterized by highly organic sediments overlying a sandy basement and well oxygenated, acid waters (pH 4.2 to 7.0) of very low conductivity (22 to 49 µm/cm2) and high transparency. The most frequent and abundant taxa collected were Chironomidae, Oligochaeta and Mollusca, which together constituted between 50% and 90% of the macrofauna collected (organisms retained on 500 µm mesh size sieve). The macrofauna was more diverse on sediments in vegetated zones, and the abundance of Chironomidae but not other benthic groups was significantly higher (Wilcoxon test, p<0.05) than in the absence of vegetation. The meiobenthos (organisms retained on 150 µm mesh size sieve) was rich and abundant specially on sandy bottoms covered with little organic matter. Common meiobenthic organisms included nematodes, harpacticoid copepods and testacea protozoan. Samples were ordinated using a principal components analysis and cluster analysis technique. Along the first principal axis, samples follow a N-S geographic gradient, probably related to an increasing development of submerged macrphytes. In the absence of submerged vegetation, the bottom of the largest lakes studied are exposed to wind action, thus permitting the frequent resuspension of sediments and providing little opportunity for the development of a benthic fauna. The second and third axis separate samples which differ in the thickness of organic sediments presented on the sandy basement. The second axis also separates lakes from streams.