Heterostyly is a genetic polymorphism of flowering plants where morphs differ reciprocally in the placement of anther and stigma, and has been reported from more than 20 families. The evolutionary maintenance of heterostyly has been postulated to be disassortative mating between morphs. Rigorous test of this “disassortative mating hypothesis” requires parental analysis with genetic markers. However, such attempts have been made only in a few species, despite the phylogenetic diversity of heterostylous species. We examined the mating pattern and other fitness components of a distylous primrose, Primula kisoana, in its natural population. Whereas we did not find a significant between-morph difference in female fitness, vegetative growth or survival, morphs showed substantial difference in siring success, with most genotyped seeds sired by short-styled morph. Therefore, we did not find the evidence for disassortative mating hypothesis. Our results, together with previous studies of the genetics of distyly in Primula, rather suggest a novel hypothesis that distyly may be maintained by overdominance. Additional studies both in P. kisoana and other heterostylous species are needed to further investigate the role of overdominance in the maintenance of heterostyly.