Summary Axel Kwet This work is based on field studies carried out between 1995 and 1999 in the eastern Araucaria forest of Rio Grande do Sul, southern Brazil. To date, 47 anuran species from 5 families have been found in this region. The whistling frogs (Leptodactylidae) with 20 species and the tree frogs (Hylidae) with 19 species and high population densities are the most speciose families. In the CPCN Pró-Mata (Centro de Pesquisas e Conservação da Natureza Pró-Mata), a 5,000 ha forest reserve, 32 anuran species were observed. Causes of species diversity in this subtropical region are discussed, and including a large variety of different habitats, the high and even annual precipitation and the availability of numerous spawning sites. A relatively clear ecological separation between the anuran species of open campos habitats (40%) and the forest (35%) was found. Few species (25%) were generalists that occurred in both habitat types. During the breeding season, about 60% of the anurans used permanent water bodies in open habitats, 40% used temporary ponds in open campos and 40% permanent forest ponds. Only 20-30% of the species used small streams of the campos or forest streams.