The germination of pearl millet (Pennisetum typhoides S. & H.) seeds was investigated at constant temperatures between 12 °C and 47 °C on a thermal gradient plate. The rate of germination increased linearly with temperature from a base Tb to a sharply defined optimum To beyond which the rate decreased linearly with temperature, reaching zero at Tm. The linearity of the response both above and below To allowed time and temperature to be combined in a thermal time at which a specified fraction of the seeds germinated. Within the population Tb and Tm were constant.