Relationships between seasonal fluctuations in pheromone trap catches of Spodoptera litura F. (Lepidoptera: Noctuidae) and the number of soybean leaves newly injured by young larvae of this species were examined in soybean fields from 1999 to 2002 in Saga Pref., Japan. In soybean fields sown in July, a small peak of injured leaves occurred in mid to late August and a large peak occurred in mid to late September. In most cases, the pheromone trap catches and occurrence of injured leaves exhibited similar trends with both peaks occurring almost simultaneously (Ϯ3 days) in mid to late September. This suggests that not only a prominent increase in soybean leaves injured by young larvae, but also a prominent increase in pheromone trap catches are useful information to determine a precise timing of control. On the other hand, the timing of control seems to be difficult to predict using the effective accumulative temperature of this species.