The persistence of intrapopulation phenotypic variation typically requires some form of balancing selection since drift and directional selection eventually erode genetic variation. Heterozygote advantage remains a classic explanation for the maintenance of genetic variation in the face of selection. However, examples of heterozygote advantage, other than those associated with disease resistance are rather uncommon. Across most of its distribution, males of the aposematic moth Arctia plantaginis have two hindwing phenotypes determined by a heritable one locus-two allele polymorphism (genotypes: WW/Wy = white morph, yy = yellow morph). Using genotyped moths we show that the presence of one or two copies of the yellow allele affects several life-history traits. Reproductive output of both males and females, and female mating success are negatively affected by two copies of the yellow allele. Females carrying one yellow allele (i.e. Wy) have higher fertility, hatching success, and offspring survival than either homozygote, thus leading to strong heterozygote advantage. Our results indicate strong female contribution especially at the postcopulatory stage in maintaining the color polymorphism. The interplay between heterozygote advantage, yellow allele pleiotropic effect and morph-specific predation pressure may exert balancing selection on the color locus, suggesting that color polymorphism may be maintained through complex interactions between natural and sexual selection.