In order to assess the dissemination of hygienically relevant fungi via the public drinking water distribution system, a 12-month survey was performed on groundwater-derived drinking water from 29 water supplies in North Rhine-Westphalia, Germany. Frequencies of contaminated water samples, and the prevalent species and patterns of occurrence in raw water, waterworks, the network and house installations were studied on the basis of 2657 water samples. Results were obtained by long-term incubation of 1 ml aliquots of water samples on agar-based culture media, following bacteriological procedures documented in the German drinking water regulations (Anon, 1990). No correlation with standard hygiene indicators, such as E. coli or other coliform bacteria was observed. Common opportunistic and allergenic Aspergillus species were encountered only rarely. The fungal flora was dominated by a limited number of species of Acremonium, Exophiala, Penicillium and particularly Phialophora; some of them occurred throughout the entire drinking water system and are thought to constitute a resident fungal flora. Phialophora sp. nov., to be described as a new species elsewhere, was ubiquitous; it was found in 26.6% of the samples positive for fungi (7.5% of 2657). Fungal diversity in the network itself was significantly lower than in raw water and house installations, indicating that not all fungi gaining access to the system are capable of surviving for longer periods. For species such as Verticillium lecanii, found exclusively after the introduction of newly buried pipes and remaining localized at those sites, introduction via arthropod vectors is likely. The resident species of Phialophora, Exophiala and Acremonium are particularly significant as they are shown to be disseminated efficiently by public drinking water.