Egg development ofHeliothis zea(Boddie) was studied under various static and fluctuating environmental regimes. Two computer-controlled environmental chambers were programmed to fluctuate temperatures through diurnal sinusoids of ±5°F (2.8°C) to ±20°F (11.1°C) using the same mean temperatures as regimes in statically controlled environmental chambers.The heat-units concept of development proved not to apply to bollworm egg development. In its stead, a hypothesis is formulated based on the egg-development curve measured in static thermal conditions.