The complex interactions between primary production, soil and soil organisms (in particular Lumbricidae and Enchytraeidae) during organic matter decomposition have developed over evolutionary time under specific habitat conditions. Thus, in order to study these processes, four forest sites in Germany (two in southern Hesse, two in eastern Saxony) and two sites in Central Portugal were selected to study both the oligochaete fauna and litter decomposition (almost all of them with a pH <4). Here, the results of the first part of this his work, i.e. the abundance and the species composition of the oligochaete fauna, are described. All samplings were performed using standard ISO-methods, with different temporal intensity. In detail, nine common Central European earthworm species were found at the six study sites plus one unidentified species at a Portuguese site. Abundance was usually low (< 10 Ind/m 2); higher numbers (72.5 and 17.5 Ind/m 2) were detected only at the two Saxonian sites. In total, 53 enchytraeid species were found; 43 of them already described. At four sites densities fell within the expected range for German deciduous forests (20 000-80000 Ind/m 2). Finally, notes on the taxonomy and biogeography of several potworm species are given.