All pathogenic Naegleria fowleri isolated from the environment were highly virulent to mice when instilled intranasally. Axenic cultivation gradually decreased virulence of highly virulent strains. This decrease was most pronounced in environmental isolates and of minor importance in N. fowleri isolated from human cerebrospinal fluid. The low virulent strains obtained by continuous axenic cultivation appeared after clonation to consist of individuals with different virulence. Virulence could be enhanced in low virulent strains by brain passage and passages in Vero cell cultures, but could not be induced by these methods in nonvirulent strains isolated from the environment. Different mice strains showed different sensitivities to infection with pathogenic Naegleria. In addition, older mice were less sensitive than younger animals to low virulent strains.