Controlled environment studies of Pennisetum clandestinum showed that at 10°C stamens of the male-fertile strain were not exserted from the floret although stigmas emerged normally. At higher temperatures both stamens and stigmas were exserted. Pollen abortion was high at 10°C and was increased by lengthening photoperiod. Flowering of the male-sterile strain was not changed by any temperature or photoperiod. These responses to temperature may explain the natural sterility during the cool season.