Rats, mice, guinea-pigs, sheep, cattle and two species of wild duck were treated daily with suspensions of the conidia of Culicinomyces sp. Treatments were contained for 4 weeks in the case of the ducks and 6 weeks for the other animals. Suspensions which are pathogenic for mosquito larvae had no observable effect on the health of the test animals. No significant changes occurred in a range of hematological and biochemical values and at autopsy no macroscopic or microscopic signs of sporulation or tissue invasion by Culicinomyces sp. were observed.