Free-water-phase and surface-associated microorganisms from drinking water were detected and roughly identified by hybridization with fluorescence-labeled oligonucleotide probes complementary to regions of 16S and 23S rRNA characteristic for the domains Bacteria, Archaea, and Eucarya and the beta and gamma subclasses of Proteobacteria. Samples of glass-attached biofilms and plankton were taken from a Robbins device installed in a water distribution system. More than 70% of the surface-associated cells and less than 40% of the planktonic cells visualized by 4',6-diamidino-2-phenylindole staining bound detectable amounts of rRNA-targeted probes. These findings are an indication for higher average rRNA content and consequently higher physiological activity of the attached microbial cells compared with the free-living cells. All detectable cells hybridized with the bacterial probe, whereas no Archaea and no Eucarya cells could be detected. Simultaneous hybridization with probes specific for the beta and gamma subclasses of Proteobacteria revealed that microcolonies already consisted of mixed populations in early stages with fewer than 50 cells. These observations provide further evidence that the coexistence and interaction of bacteria in drinking water biofilms may be an integral part of their growth and survival strategies.