Negative frequency–dependent selection is a major selective force maintaining sexual polymorphisms. However, empirical demonstrations of frequency–dependent reproductive success are rare, particularly in plants. We investigate this problem by manipulating the frequencies of style morphs in a natural population of Narcissus assoanus, a self–incompatible herb with style–length dimorphism and intra-morph compatibility. We predicted that the reproductive success of morphs would vary negatively with their frequency because of the effects of morph–specific differences in sex–organ position on patterns of pollen transfer. This prediction was generally supported. The fruit and seed set of the two morphs did not differ significantly in plots with 1 : 1 morph ratios. However, short–styled plants produced significantly fewer seeds than long–styled plants in monomorphic plots, and significantly more seeds than long–styled plants in plots with ‘long–biased’ morph ratios. These patterns indicate that in the absence of physiological barriers to intra–morph mating, negative frequency–dependent selection contributes to the maintenance of stylar polymorphism through inter–morph pollen transfer. Our experimental results also provide insights into the mechanisms governing the biased style–morph ratios in populations of Narcissus species.