Fecundity and egg size were compared in adult of the females adult diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae) treated and untreated with sublethal doses of fenvalerate in order to understand the physiological significance of insecticidal hormoligosis. The 4th stadium larvae were treated with fenvalerate at LD 25 or LD 50. More eggs were laid by treated females at LD 25 than untreated females, although the difference was only marginally significant (pϭ0.07). However, the eggs laid by treated females at LD 25 and LD 50 were significantly smaller in size than those laid by the control. The reproductive effort (fecundityϫegg size) did not differ between treated females and the control. The treatment of a sublethal dose (LD 50) against the parent affected the development and survival of offspring at immature stages of males. The hatchability of smaller eggs laid by treated females at LD 25 tended to be lower than those of controls under different humidity conditions, and the difference was apparent at a humidity of 29%. The survival rate of the offspring at immature stages was lower in the treatment group (LD 25) than in the control group, and development tended to be prolonged in the former group at temperatures higher than 20°C.