The conflicting reports on the influence of light on the germination of A. fatua L. (wild oat) seed may be due to the use of seed of different types and of differing dormancy status. In an attempt to resolve these uncertainties we tested 30 stocks of seed of known type and dormancy status. Light promoted the germination of three types of partially-dormant seed but had no effect on fully-dormant and fully non-dormant seed. The magnitude of the promotory effect was similar in all stocks tested irrespective of the level of dormancy. Following the application of dormancy-breaking treatments germination of fully dormant seed was also promoted by light. The photoreceptor involved was shown to be phytochrome.