Transgenic Bollgard II cotton, Gossypium hirsutum L., expresses Cry1Ac and Cry2Ab proteins that provide control of lepidopteran larvae, including Helicoverpa and Heliothis species (Lepidoptera: Noctuidae) worldwide. Experiments conducted at Katherine, Northern Territory, Australia evaluated the impact of night minimum temperatures on Cry1Ac and Cry2Ab protein levels in Bollgard II cotton. In both 2003 and 2004, potted plants were either grown outside continuously or protected from cold in a glasshouse each night. In 2003, bulked samples of leaves were taken after two periods of low minimum temperature and used to determine a cold-stress threshold and critical period. In 2004, replicated samples were taken on 10 dates spanning five periods of low minimum temperature, allowing analysis of seasonal variation in Cry protein levels. The protein level was markedly higher for Cry2Ab than for Cry1Ac. Cry1Ac protein level peaked midseason and was not adversely affected by minimum temperatures down to 2.6 degrees C. The Cry2Ab protein level remained approximately constant but was reduced by low minimum temperatures (threshold, approximately 14 degrees C) for up to 6 d after each chill. The rate of Cry2Ab protein loss was 1.15 and 1.01% per chilling day-degree below threshold in 2003 and 2004, respectively. Impact would seem to be negligible on both the overall efficacy against lepidopteran larvae in-crop and on the current pyramided genes/high-dose/refuge Bt resistance-management strategies because the cold-stress effect is transient, a high level of Cry2Ab protein is still expressed, and there is no impact of chilling on Cry1Ac protein level.