Daily patterns of activity and the effects of adult density and exposure to host plants on the onset of reproductive activities and age-specific fecundity of the diamondback moth,Plutella xylostella(L.), were determined under constant conditions of 21°C, 70%RH, and a 16:8 (L:D) photoperiod. More than 50% of adult emergence occurred in the first 8 h of the photophase. Although calling occurred throughout the scotophase,>70% of mating and oviposition took place in the first 4 h. Approximately 90% of females had begun to call, mate, and lay eggs, and the females contained ≥1 mature egg by night 4 when kept with males and host plants in individual pairs. In contrast, only 27% of females kept with males but without host plants had mated by night 4, and only 64% contained mature eggs. Individually caged females laid eggs at a rate>50% of their maximum daily oviposition on nights 3–5, and for females caged in groups of 50 males and 50 females this occurred on nights 4–10. Ovarian development also was consistently slower in the group-caged females than in the individually caged females, the former containing 16 to 93% as many mature eggs as the individually caged females, depending on age and prior exposure to plants. Male sexual maturation was not complete at adult emergence, and only 54% of day-l males were able to fertilize 4-d-old, virgin females. By night 4,98% of males were able to do so regardless of whether or not they were exposed to host plants. Females mated to day-4 males that were exposed to plants for the first three nights produced significantly more viable eggs compared with females mated to day-4 males that were not previously exposed to host plants.