Many organisms exhibit polymorphism in various traits. They are important to understand how distinct phenotypes are maintained in natural populations. Persicaria lapathifolia usually has no trichomes on its leaf surface (glabrous type), but some (hairy type) bear them, which act as a physical defense against a leaf beetle Galerucella grisescens. The hairy type produces fewer seeds than the glabrous type. We studied maintaining mechanism of the dimorphism in trichome production in P. lapathifolia focusing on environmental heterogeneity (EH) represented by the density of herbivores, and negative frequency dependent selection (NFDS). In a field experiment with different frequencies of the two types, the glabrous type produced more seeds than the hairy type regardless of the frequency of both types, suggesting that NFDS is not the main factor. In a transplanting experiment in two localities, one with high density and the other with low density of G. grisescens, the glabrous type produced more seeds than the hairy type in both localities. This is probably because the density of G. grisescens was very low in the study year in both localities and the density of G. grisescens may be a critical factor affecting the relative frequency of both types.