In the course of screening for insecticidal metabolites of fungi, a new insecticidal cyclodepsipeptide, bassianolide (1), and the known compound beauvericin were isolated from mycelia of Beauveria bassiana, a pathogenic fungus for various insects. The structure of 1 was determined as a cyclodepsipeptide composed of four moles each of L-N-methylleucine and D-α-hydroxyisovaleric acid (1a). Fifth instar larvae of silkworm (Bombyx mori) were killed when fed with an artificial diet containing 1 at a dose of 13 ppm, while beauvericin was not lethal even at 1000 ppm. Bassianolide was also isolated from mycelia of Verticillium lecanii, another insect-pathogenic fungus.