Beauveria Bassiana (Bals.) Vuill. isolated from infected Colorado potato beetle larvae is shown to be a pathogen to this host. The experiments indicated that some of the infected larvae may pass through the larval stage, but owing to further mycelial growth finally succumbed in the pupal stage. The fungus does not attack the eggs, but infected young larvae were found dying on the fourth day after hatching, when the eggs had been dusted with B. Bassiana spores. The mycelial growth on the infected larvae is more rapid in the soil than above ground.