Pathogenic free-living amebas may produce fatal infection of the central nervous system known as Primary Amebic Meningoencephalitis (PAM). In Richmond, Virginia, 17 cases have occurred, more than in any other location in the world. The objectives were to examine freshwater lakes in the Richmond, VA area for pathogenic amebas, particularly Naegleria, and gain some understanding of conditions which might affect their occurrence. Nine lakes with different limnological characteristics were sampled biweekly. Ameboflagellates consistent with Naegleria were often isolated from all lakes, but pathogenic ones were found infrequently. The reason for this finding appeared to be temperate climate and short periods of time (days to weeks) that sustained elevations of water temperature (>26C) occurred. No quantitative relationship of coliform bacteria to pathogenic amebas or ameboflagellates was found, but Pseudomonas fluorescens and Serratia marcescens appeared in the laboratory to be inhibitory to pathogenic Naegleria. In virtually every lake from which Ps. fluorescens was consistently isolated, no pathogenic Naegleria were found. The majority of pathogens and the most virulent organisms were isolated from the North Anna Reservoir complex which contained several coolant lagoons and canals for a nuclear power plant. Studies of this complex suggested that if natural fresh waters are artificially heated for a sustained period of time, pathogenic Naegleria may be selected or concentrated, even in a cold or hostile climate, possibly serving as a source for contaminating other nearby waters. Pathogenic Naegleria were isolated only with thermal enrichment, but not before or after.