In the laboratory, second- and third-instar Plutella maculipennis (Curtis) larvae were about equally susceptible to parasitism by Diadegma insularis (Cresson) or by Microplitis plutellae Muesebeck. The fourth and last instar was less susceptible than the second and third. Larvae of the first instar, although usually within the tissues of the food plant, could be parasitized. Maximum progeny obtained from female parasites during a lifetime, with regularly renewed supplies of hosts, averaged 516 in D. insularis, and 232 in M. plutellae. Effectiveness of D. insularis among a given number of host larvae improved slightly, reaching 85/100 as cage size increased from 50 to 200 sq. in., and declined somewhat, from 124/200 to 94/200, when cage size was further increased from 200 to 1500 sq. in. In field cages of 1, 2, and 4 sq. yd, with 200 host larvae, single female D. insularis parasitized about 35, 20, and 7% of hosts respectively. With 50, 100, 200, and 400 host larvae in 200-sq.-in. cages, and one female per unit, average progeny of D. insularis were respectively 39, 70, 113, and 150; of M. plutellae, approximately the same. In competition, two females, one of each species, operating among the same lot of larval hosts, produced more total progeny than two of the same species. Increasing constant temperatures from 20° to 30° favoured D. insularis slightly, but viability and effectiveness of M. plutellae at 30° was reduced.