The life cycle of Lethocerus deyrollei (Vuillefroy) in Sayo-cho, Hyogo, Japan (35 °00′ N, 134 °21′ E), was studied in terms of flight, reproduction, and energy allocation as well as developmental zero (t₀) and required heat units (K) for a generation. Counts of bugs attracted to electric lighting showed that L. deyrollei flew most frequently in July, the second half of the mating season. The weight of flight muscle showed no significant yearly change despite a conspicuous difference in the flight activities. Fat content in the abdomen was low during summer, when oviposition activity reached a high level in females. Thereafter, lipid accumulation increased to mid-October when flight to an overwintering site took place. The lipid reservoir in late summer may be critical for surviving winter but no significant decrease was observed during overwintering. The number of eggs in the ovary indicated univoltism because no females had mature eggs late in the year. t₀ and K were calculated based on laboratory rearing at different temperatures. Thus, estimated K (635 degree days) and local heat mass suggest a univoltine life cycle in Sayo-cho. The possible areas for successful inoculation of L. deyrollei were examined on the basis of available heat mass and day length in places where the species is extinct or not observed. Based on the estimated K and critical day length for reproductive maturation, Niigata and Tokyo fulfill suitable conditions of the Sayo population to complete a univoltine life cycle.