Historical data on the biodiversity of oligochaetes inhabiting ground waters of Slovenia depicted a fauna of 25 species, 19 of which are stygobiotic. Over about the last 35 years, faunistic surveys carried out in Slovenian ground waters has enabled us to conduct extensive studies on the oligochaete fauna of this environment. Three primary sources of information have been integrated to summarize in this paper: a campaign in Slovenian caves conducted by Fabio Stoch, a large collection of groundwater fauna made available to us by Boris Sket, and samples collected during the European project PASCALIS. The data derived from the examination of this large amount of material has enabled us to broaden our knowledge of the oligochaete diversity of Slovenia, increasing the number of species to one hundred, and has allowed us to summarize the biological diversity in Slovenian waters to be a substantial percentage of the known diversity present elsewhere in Europe. Endemic, rare and new species constitute a remarkable proportion of the stygobiotic oligochaete fauna. Among these, species of the genera Trichodrilus, Rhyacodrilus, Rhyacodriloides, Parvidrilus, Epirodrilus and Abyssidrilus are some of the most noteworthy taxa because of their endemicity, range-size, rarity, habitat selection, and/or taxonomic isolation (including phylogenetic relictuality).