1.Genotypic diversity in plant populations is known to enhance plant performance and ecosystem function. Nonetheless, the effect of genotypic diversity has rarely been examined across a population's lifecycle despite the expectation that changing conditions, such as population density, will alter the benefits of diversity. 2.We simultaneously manipulated a component of genotypic diversity (richness, the number of genotypes) and density of common evening primrose Oenothera biennis to address the consequences for herbivory and lifetime fitness in a two-year field experiment that spanned seed germination to life-time fruit production. We genotyped >1100 seedlings with microsatellite DNA markers to determine realized diversity and density in plots sown with O.biennis seeds. Our design achieved quantitative variation in plant density and diversity, with one to 44 individuals established in field plots and two to eight genotypes per polyculture plot (based on microsatellite analysis of reproductive plants). 3.We found a strong interaction between seed density and genetic diversity, with germination and establishment being 24% higher in genetic polycultures than monocultures, but only at low seed density. At high seed density, the opposite pattern emerged, with polycultures having 12% fewer individuals established than monocultures. Initial effects of emergence on plot density persisted through to the fruiting stage. 4.Higher plant densities result in increased mortality, decreased probability of reproduction, decreased plant height, and lower levels of life-time fruit production per plant. Increasing genotypic diversity increased the probability of reproduction overall, and showed a significant interaction with plant density mitigating the negative effects of high density on individual height and lifetime fruit production. 5.Synthesis. Plant density and genotypic diversity interacted from the very earliest stages of seed germination and establishment of O. biennis. This effect persisted over the two-year life-cycle of plants, and genotypic diversity buffered against the negative fitness consequences of high plant density. These results imply a dynamic interplay between the long-held paradigm of density effects in plant ecology and the genetic structure of populations.