Sperm competition is a major evolutionary force driving diversity in sperm morphology. Such sperm competition exists in C. elegans where larger male sperm consistently outcompetes hermaphrodite sperm, and larger male sperm outcompetes smaller male sperm. Variation in male sperm size among different wild isolates of C. elegans may thus have evolved in response to different degrees of sperm competition. To characterize natural genetic variation in C. elegans male sperm size, we have quantified this phenotype in a collection of 100 genetically-divergent C. elegans wild isolates. This analysis revealed considerable variation in male sperm size, however, a Genome-Wide Association Mapping Study did not allow detection of potential genomic regions underlying this variation. Surprisingly, however, this survey revealed significant differences in male sperm size of different strains derived from the N2 reference strain, pinpointing potential candidate genes involved in sperm size regulation. We are currently exploring how specific candidate genes affect spermatogenesis and sperm maturation, to ultimately define molecular events controlling sperm size variation. In addition, we will present data on the co-evolution of male and hermaphrodite sperm in C. elegans, C. briggsae and C. sp. 11; and we further discuss interspecific evolution of sperm size in gonochoristic Caenorhabditis species, which have been shown to exhibit a significantly larger mean sperm size than males of androdioecious species. We have extended these comparative analyses by integrating newly discovered species in the context of a recently established molecular phylogeny. This analysis indicates rapid evolution of sperm size and the repeated, independent evolution of exceptionally large sperm.