Field observations and an experiment were conducted to clarify the seasonal cycle of Alydus calcaratus (Heteroptera: Alydidae) in the forest-steppe zone of Russia and to test the role of day length in the control of seasonal development of this species. Insects were reared under a constant photoperiod and at natural temperature. Two alternative photoperiodic regimes were used: short (14L-10D) and long (18L-6D) days. No significant differences in duration of nymphal development were observed between these conditions. Females laid eggs under both photoperiods, although preoviposition period was significantly shorter under long-day (11.6 ± 1.4 days) than under short-day (13.8 ± 1.5 days) conditions. Eggs were kept under the same photoperiodic conditions at a constant temperature of 24°C for two months. In the long-day regime, nymphs from 37.4% of eggs hatched on 14.1 ± 1.3 days after oviposition (hatchability in the egg samples laid by one female varied between zero and 85.7%). During the same period, no nymphs hatched from the eggs laid and maintained under short-day conditions. These observations suggested that day length plays a principal role in diapause induction in this species.