In the free-living rhabditid nematode Caenorhabditis elegans, sperm size is a determinant of sperm competitiveness. Larger sperm crawl faster and physically displace smaller sperm to take fertilization priority, but not without a cost: larger sperm are produced at a slower rate. Here, we investigate the evolution of sperm size in the family Rhabditidae by comparing sperm among 19 species, seven of which are hermaphroditic (self-fertile hermaphrodites and males), the rest being gonochoristic (females and males). We found that sperm size differed significantly with reproductive mode: males of gonochoristic species had significantly larger sperm than did males of the hermaphroditic species. Because males compose 50% of the populations of gonochoristic species but are rare in hermaphroditic species, the risk of male-male sperm competition is greater in gonochoristic species. Larger sperm have thus evolved in species with a greater risk of sperm competition. Our results support recent studies contending that sperm size may increase in response to sperm competition.