A first study was made on the microbial community composition of the Indonesian crater lake Kawah Ijen (pH < 0.3) and the Banyupahit-Banyuputih river (pH 0.4-3.5) originating from it. Culture-independent, rRNA gene-based denaturing gradient gel electrophoresis was used to profile microbial communities in this natural and ancient, extremely acidic environment. Similarity in community profiles of the different sampling locations was low, indicating heterogeneity in community composition. Archaea were present at all sampling locations; archaeal diversity was low at the most acidic locations and increased at pH >2.6. Bacteria were not detected in the water column of the crater lake, but were found at all locations along the acidic river. Bacterial diversity increased with increasing pH. Eukarya were only present at pH >2.6. Retrieved rRNA gene sequences of Bacteria and Archaea were not closely related to known acidophilic species. It is concluded that tolerance to extreme acidity in this system is developed most extensively among Archaea. The acidity gradient of the Banyupahit-Banyuputih river has a clear effect on microbial community composition and biodiversity.