Under controlled environment conditions rice variety IR8 was subjected to night temperatures of 15, 20 and 25°C during each of the three periods (a) sowing to floral initiation, (b) initiation to panicle emergence, and (c) panicle emergence to maturity, with day temperatures of 30°C throughout. Night temperatures of 15°C prevented floral initiation throughout the experiment, an effect partly overcome by transfer to higher temperatures after 102 days but not without yield loss. The optimum night temperature was 20–25°C, with some evidence of change of temperature effects, as distinct from the effect of actual temperature levels. As IR8 was developed under relatively stress-free conditions, its sensitivity to temperatures below 15°C may be greater than in varieties developed elsewhere.