Trifolium repens is a species polymorphic for cyanogenesis. The polymorphism is caused by variation in two genes: Ac regulates the presence/absence of the cyanogenic glucosides linamarin and lotaustralin; Li is the structural gene for linamarase. Both genes also affect vegetative and reproductive characters of the plant. The female reproductive fitness of acac plants is about double of that of Acac plants. The male reproductive fitness was estimated by the proportion of Ac- and Zi-plants in the progeny of acaclili plants in an experimental population with known frequencies of the cyanotypes. A significant excess of Acac plants was found in the progeny. The difference in male and female fitness is one of the many genetic and environmental factors that regulate the polymorphism for cyanogenesis that is characteristic for most populations of T. repens.