The cold-hardiness of nymphal stages 1 to 5 and adult male and female Blatta orientalis was tested at 2°, −5° and −10 °C. The LT50 (time) of insects exposed to −5° ranged from 0.21 to 0.43 days. Acclimation at 10 °C for times varying up to 14 days progressively increased cold-hardiness. A 14 day acclimation at 10° prior to exposure at −5°C increased LT50 (time) to 1.1–4.2 days for the various stages; prolonging the acclimation time to 28 days produced no further increase in LT50. All stages were rapidly killed at −10 °C (LT50<0.04 days) and survived prolonged exposures at 2 °C (LT50s from 16 to > 42 days) following acclimation. The potential for survival of outdoor populations of B. orientalis over winter is discussed.