The biology of the immature stages and adult eclosion of Ufens principalis Owen, an important parasitoid of Homalodisca eggs in southern California, were studied. The duration of the egg, larval and pupal stages at 26.7°C were 0–1, 7 and 9 days, respectively. Sacciform larvae, which developed gregariously within host eggs, were motile until about five days of age, and then became sessile. Parasitized host eggs changed from whitish and soft when freshly-laid to yellow-orange and hard at five days and older. This change was accompanied by formation of septal walls separating the mature larvae and pupae. The rate of immature development had a strong positive linear relationship (R2=0.853, n=98) with temperatures in the range of 20.0–30.3°C. The theoretical minimum threshold for immature development was 13.5°C, and the required heat units were 241.0 degree-days. Adult eclosion from host eggs occurred mostly (85%) on the first two days of emergence. Although most females emerged during the morning hours (0600–1200 h), males tended to emerge earlier than females with equal emergence during the morning and late night hours (2400–0600 h). The rate of successful adult emergence was high (88%). The ratio of eclosed adults to the number of exit holes was 1.18, indicating that most adults tended to independently cut their exit holes. The number of exit holes had a strong negative relationship (R2=0.711, n=125) with exit hole size, suggesting that larger numbers of developing immatures per host egg result in an overall decrease in adult size.