The discharges of 16 thermal polluting factories were examined for the occurrence of Naegleria fowleri, the causative agent of primary amebic meningoencephalitis. Seven of these waters were shown to harbor this ameba. Of 22 N. fowleri isolates, 3 were highly virulent for mice when inoculated intranasally. The three pathogenic strains were isolated from three different places during summer. More N. fowleri were found during summer than in winter, which could be attributed to the lower temperature during winter. The distribution seems to be bound to the cooling waters of older factories and is not restricted to one type of factory. There are strong indications that the isolation of nonpathogenic seropositive strains from water is an indication for the occurrence of pathogenic N. fowleri.