none Investigations were conducted on how changes in the sterol composition of Heliothis zea (Boddle) affected the growth, development, and sterol composition of its hymenopterous endoparsitoid Microplitis demolitor Wilkinson. When the amount of sterol in the hemolymph of the H. zea larva was decreased by the addition of cholestyramine to its diet, the growth of the hemolymph-feeding parasitoid was inhibited. The larval stage of the wasp in this host was significantly longer than that of the control, and the adults that eventually developed were significantly smaller. In contrast, when the amount of sterol in the hemolymph of the H. zea larvae was increased by the addition of niacin to its diet, the growth of the parasitoid was normal, probably because its essential dietary sterol was readily available. When the H. zea larva was fed DELTA-0-, DELTA-57- or DELTA-7-sterol in place of DELTA-5-sterol, the sterol composition of its tissues was altered and reflected that of its diet. Such changes in the tissue sterols of the host did not affect the growth and development of M. demolitor, although the sterol composition of the parasitoid was also altered. These studies indicate that the quantity rather than the structure of the sterols in the hemolymph of H. zea may help determine whether M. demolitor develops successfully in this host.