In aqueous environments bacteria can occur planktonically in the water phase, or associated in biofilms attached to solid surfaces or other phase boundaries. Plankton organisms in surface waters provide external surfaces which can be colonized by biofilms. Plankton surface can comprise areas up to thousands of km2 in a lake. Possible associations of potentially pathogenic bacteria with phyto- and zooplankton were observed in a field study in a freshwater environment (Lake Baldeney, Essen, Germany) and in laboratory experiments. Hygienically relevant microorganisms considered were, bacteria with faecal origin (Escherichia coli, coliforms, intestinal enterococci, Clostridium perfringens), an obligate human pathogen of faecal origin (Campylobacter spp.), and environmental opportunistic bacteria (some coliforms, Pseudomonas aeruginosa, Aeromonas spp., Legionella spp.). For all of the investigated hygienically relevant bacteria an association with freshwater plankton could clearly be demonstrated, except for Campylobacter spp. which was only found in water and Legionella spp. which was not detected by culture. The bacterial abundance in all samples was found to be higher with plankton than compared to the free water. With the culture-independent methods, fluorescence in situ hybridization (FISH) and quantitative polymerase chain reaction (qPCR), the organisms P. aeruginosa and Legionella spp. were found in significant higher concentrations in water and plankton than with cultural methods. This observation indicates that both may occur in a viable but nonculturable (VBNC) state. In laboratory experiments hygienically relevant bacteria were co-cultivated with Daphnia magna in microcosms. To study cladoceran-bacteria associations in detail, it was discriminated between carapace-associated and ingested bacteria. The organisms P. aeruginosa and A. hydrophila were found more frequently located on the carapace of D. magna, whereas most of E. faecalis was located in the gut. FISH analysis indicated the possibility of VBNC cells for these organisms in association with D. magna. The field study as well as laboratory microcosms indicate that there are associations and accumulations of pathogenic bacteria, with different characteristics and origins, with plankton. Plankton can act as a reservoir and a vector for potentially pathogens and may spatially enhance bacterial concentrations up to infectious doses. In case of VBNC bacteria, it is possible that the VBNC cells resuscitate and regain their virulence. Plankton-pathogen associations are of relevance considering human health in drinking water production and recreational use of the surface water