In most invertebrate taxa, males are smaller than females and large male sizes are only favoured when male–male competition for access to females is intense. In addition, in species where sperm competition is important, relatively large investments in testis growth and ejaculate size will also be selected. The use of copulatory plugs by males of some taxa, however, can prevent sperm competition. We hypothesized that, across related species in which males use plugs, relative testis size would decrease with increases in the severity of male–male competition. We tested this prediction across 112 species of acanthocephalan parasites, worms that use copulatory plugs and in which male–male competition occurs. As a measure of the intensity of male–male competition in a given species, we used sexual size dimorphism. Male and female body volume covaried allometrically, suggesting that sexual size dimorphism in acanthocephalans is the product of sexual selection. Our main finding is that relative testis volume, corrected for male body volume, decreases significantly as male body volume relative to female volume increases, i.e. as the sexual size dimorphism becomes less female-biased. All our results remained unchanged after we controlled for potential phylogenetic effects. The relationship indicates that investment in testis growth beyond the minimum size required for efficient fertilization becomes increasingly less important in species where males appear to compete intensely for mating opportunities.