Acidic aquatic ecosystems are mainly characterized by low pH and high concentrations of metals and other elements with evident effects on local community structure. Acidity effects on benthic communities in one of the world's largest extremely acidic crater lakes, the Kawah Ijen (East Java, Indonesia) (pH < 0.3), and the acidic Banyupahit-Banyuputih river (pH 0.7-3.3) originating from it, had not been investigated. Therefore, macroinvertebrate and algal community structure in the acidic river were studied and compared to the neutral rivers Kali Sat and Kali Sengon in the same catchment. The aquatic foodweb of the Banyupahit-Banyputih river was poorly developed and a complete lack of macroinvertebrate species was observed at pH values below 2. At two sampling stations with pH 2.69-3.47 and pH 2.26-2.89 only chironomids were found and other acid-tolerant macroinvertebrate taxa from acidic aquatic systems reported in the literature were absent. No statistically significant differences of metal body burden in chironomids could be detected between locations. In contrast, algal communities were more diversified and the number of taxa increased with increasing pH. Remarkably, high densities of an unknown green alga were found in samples from the acidic crater lake. Diatom communities were characterized by dominance of the acid tolerant Pinnularia acoricola at the most acidic sites, and a gradual replacement by other species with increasing pH. Particular features of the river, such as the lack of aquatic macrophytes and the presence of acid-tolerant algal species (e. g. Euglena mutabilis, Achnanthes minutissima), revealed not only the strength of the acid selection on these communities, but also the role of indirect physical and chemical factors (e.g. high Al, low P concentrations). It is concluded, therefore, that the ability of the studied macroinvertebrates and algal species to withstand the acid circumstances is not only due to their resistance to low pH, but also to the consequences of it. © 2006 E. Schweizerbart'sche Verlagsbuchhandlung.