Aspects of the biology of the adult stage of Ufens principalis Owen (Hymenoptera: Trichogrammatidae), an important parasitoid of Homalodisca (Hemiptera: Cicadellidae) eggs in southern California, were studied. The field-based sex ratio of U. principalis was significantly different from 1:1 and was female biased (62% females). Females and males did not differ significantly in size based on measurements of head width and hind tibia length, and these two variables were highly correlated. There was a significant effect of female age on egg load. Females aged 1, 4, and 5 d had significantly higher egg loads than freshly emerged females. There was also a significant positive linear relationship between egg load and hind tibia length. Egg load averaged 37.9 ± 1.1 eggs. Eggs, which measured 207.8 ± 12.2 µm in length, were elliptical with an anterior nipple-shaped constriction. The longevity of adults was significantly affected by temperature and food (1:1, honey/water). Food had a greater effect in prolonging life as temperature decreased from 35.0 to 10.0°C. The most long-lived adults were fed on honey and kept at 10.0°C (20.2 ± 3.3 d). The rate of parasitism was significantly affected by host age, being significantly higher for 0-h-old eggs than for 12-24-h-old host eggs. There was no significant difference in parasitism between 0-h- and 0-6-h-old eggs and between 0–6 and 12-24-h-old eggs.