Within tested strains of the genera Naegleria and Acanthamoeba the ability to grow at high temperatures seems directly related to virulence, with nonvirulent strains unable to grow at normal or elevated body temperatures. Outside these genera, nonvirulent Hartmannella and Tetramitus do grow at elevated temperatures, which suggests a barrier to pathogenicity other than temperature sensitivity. The high optimal temperature of pathogenic Naegleria apparently explains previous difficulty in obtaining isolates from the aquatic environment.