ABSTRACT: Despite experimental evidence for effects of primary producer diversity and consumer species diversity on population and community processes, little is known about how diversity at these multiple trophic levels may interact. We conducted a mesocosm experiment to examine the independent and interactive effects of seagrass Zostera marina genotypic diversity and grazer species diversity on seagrass, epiphyte, and grazer responses. There were no interactions between seagrass genotypic diversity and grazer species diversity, per se; however, the effects of seagrass genotypic diversity on both seagrass and grazer biomass depended on grazer species identity. In particular, seagrass biomass was higher in polyculture than in monoculture, but only when the sea hare Phyllaplysia taylori was the only grazer present. This enhanced growth was due to complementarity among genotypes in the presence of P. taylori. Seagrass genotypic diversity effects on grazer biomass and fecundity were small and/or idiosyncratic, yet grazer species composition had a large impact on grazer reproductive effort. Only grazer species identity, and not seagrass genotypic diversity or grazer species diversity, affected epiphyte biomass, consistent with other findings of the importance of species identity. Our results confirm the effects of seagrass genetic diversity on the plant itself as well as the grazer species that utilize it for both food and habitat. Furthermore, they emphasize the importance of grazers for controlling epiphyte and seagrass biomass.