The effect of photoperiod on preimaginal development and diapause induction (for reproductive diapause) was evaluated for two geographic populations of the predator Geocoris punctipes (Say). One population was from Lexington, KY (N38° 04′, W84° 29′), and the other from Tifton, GA (N31° 28′, W83° 31′). Six photoperiods were used in the experiment (all at 24 ± 1°C): 10:14, 11:13, 12:12, 13:11, 14:10, and 15:9 (L:D) h. Photoperiod and population had significant effects on the developmental times of embryos and nymphs. However, the developmental differences had no apparent relationship with photoperiod, and those between populations were slight (<1 d). The incidence of diapause increased for both populations as photoperiod decreased, but at daylengths shorter than 14:10 (L:D) h, the frequency of diapause increased much more steeply in the Kentucky population than in the Georgia one. At its peak, the incidence of diapause in the Georgia population (40.9%; 10:14 [L:D] h) was about half that of the peak of the Kentucky population (81.8%; 12:12 [L:D] h). Also, the critical photoperiod for diapause induction was ≈1 h longer in the Kentucky population than in the Georgia population. The preoviposition period was notably affected by photoperiod, and this effect differed between populations. Short daylengths prolonged the preoviposition period in both populations, but the contraction of the preoviposition period in response to longer daylength was more pronounced in the Georgia population. The considerable difference in diapause incidence between the two populations may reflect the relative predictability of inclement conditions in the respective locations.