AbstractA proportion of the maize seed sold in the Netherlands is genetically impure. The most common form of genetic impurity in hybrid seed production is the accidental self‐pollination of the female parent. Varying quantities of selfed seeds were added to certified seedlots of three‐way cross hybrids. These mixtures were tested in nine field experiments, in 1985 and 1986.Whole plant dry matter yield decreased by 40 to 45 kg/ha per percent of selfed seed added. The effect of selfed seed on dry matter yield did not differ significantly in the hybrids stud