During 1962, barnyardgrass (Echinochloa crusgalli (L.) Beauv.) seedlings were infested by the larvae of Hyperodes humilis Gyll. The larvae attacked the growing points and the youngest tissues in the crown as well as meristematic tissues of intercalary growth. These infestations produced considerable decreases of barnyardgrass growth and yields, suggesting the possibility of effective biological control of barnyardgrass.