Vibrio cholerae non-O1/non-O139 strains have caused several cases of ear, wound, and blood infections, including one lethal case of septicemia in Austria, during recent years. All of these cases had a history of local recreational activities in the large eastern Austrian lake Neusiedler See. Thus, a monitoring program was started to investigate the prevalence of V. cholerae strains in the lake over several years. Genetic analyses of isolated strains revealed the presence of a variety of pathogenic genes, but in no case did we detect the cholera toxin gene or the toxin-coregulated pilus gene, both of which are prerequisites for the pathogen to be able to cause cholera. In addition, experiments were performed to elucidate the preferred ecological niche of this pathogen. As size filtration experiments indicated and laboratory microcosms showed, endemic V. cholerae could rapidly grow in a free-living state in natural lake water at growth rates similar to those of the bulk natural bacterial population. Temperature and the quality of dissolved organic carbon had a highly significant influence on V. cholerae growth. Specific growth rates, growth yield, and enzyme activity decreased markedly with increasing concentrations of high-molecular-weight substances, indicating that the humic substances originating from the extensive reed belt in the lake can inhibit V. cholerae growth.Vibrio cholerae is both a human pathogen and a natural inhabitant of aquatic environments (10, 13). More than 200 serogroups have been identified to date, but only serogroups O1 and O139 are associated with epidemic cholera (48). Non-O1/non-O139 strains have so far not been found to be involved in epidemic cholera but can cause other diseases in humans. Since the seminal work of Colwell et al. (10), several investigations have traced the potential ecological niches where V. cholerae thrives and survives in aquatic environments, but still, the ecology of this human pathogen is poorly understood (13, 60). V. cholerae has been shown to live mainly in association with crustacean zooplankton (18, 23) and has been detected with algae (16, 27, 28) in a variety of aquatic environments where it is involved in surface biofilm formation (55) and where it degrades the polymeric substances chitin (40) and mucilage (49). In addition, V. cholerae has been isolated from freshwater and marine macrophytes (26) as well as from benthic animals like prawns, oysters (52), crabs (3), and chironomid egg masses (5) and has also been shown to be able to replicate intracellularly in free-living amoebae (1). In contrast to the fact that V. cholerae can grow in the particle-associated state, a few reports have demonstrated that V. cholerae can also grow in water as a free-living organism in the planktonic phase (39,53,60). Thus, the existence of at least two main growth strategies of environmental V. cholerae (particle associated versus free living) can be assumed, which has important consequences for mechanisms controlling population size and survival. Food web interactions,...