Previous workers have obtained evidence suggesting that, under natural conditions, Papaver dubium reproduces by mixed selfing and random mating, the contribution of selfing being substantial. In order to obtain a more quantitative estimate of the amount of selfing, a small number of plants, homozygous or heterozygous for the recessive (flower colour) mutant, magenta, were raised among a large number of wild-type homozygotes and allowed to open pollinate. Progenies raised from these recessive homozygotes and from the heterozygotes, provided estimates of the degree of selfing. Results showed that the degree of selfing varies significantly, both between plants and between different flowers on the same plant. The estimates of average selfing obtained lay between 71 and 81 per cent, the estimates being about the same in two different years. In view of these and earlier results, it was concluded that a high degree of selfing obtains in natural populations of P. dubium; the degree of selfing being sufficiently large to have an important effect on the genetical structure of such populations.