Germination of Bromus sterilis L. seeds is unusual in that it is inhibited when the photoreceptor phytochrome exists as Pfr, the degree of inhibition being dependent on the proportion of Pfr established. However, inhibition of germination is transitory and at 15 °C is evident only at l.5 d from sowing. Although germination is reduced at temperatures lower than 15 °C, Pfr is nevertheless inhibitory to germination; inhibition persisting far longer than at 15 °C C. Temperatures higher than 15°C C are also inhibitory to germination but at these temperatures there is no additional inhibitory effect mediated by Pfr. Under conditions of low moisture status the inhibitory effect of Pfr is also markedly increased. The results are discussed in relation to the role of light in the germination behaviour of B. sterilis seeds in the natural environment.