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 LD₂₅ or LD₅₀. More eggs were laid by treated females at LD₂₅ than untreated females, although the difference was only marginally significant (p=0.07). However, the eggs laid by treated females at LD₂₅ and LD₅₀ were significantly smaller in size than those laid by the control. The reproductive effort (fecundity×egg size) did not differ between treated females and the control. The treatment of a sublethal dose (LD₅₀) 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 LD₂₅ 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 (LD₂₅) than in the control group, and development tended to be prolonged in the former group at temperatures higher than 20°C.