Sperm competition is widespread and influences both male investment in spermatogenic tissue and ejaculate characteristics. Sperm competition models assume trade-offs between sperm size and number, although such trade-offs may be difficult to detect. This study examines the effects of sperm competition risk on the sperm production characteristics of the freshwater snail Viviparus ater. In this prosobranch, females mate frequently and store sperm, generating sperm competition. Males produce two sperm morphs, fertile eupyrene sperm and non-fertilizing oligopyrene sperm. Non-fertilizing sperm may play a role in sperm competition and therefore, like fertilizing sperm, the number produced could vary relative to sperm competition risk. In addition; trade-offs between sperm number and sperm size may be expected. We manipulated the sex ratio of sexually mature snails and found the presence of rivals affected the ratio of oligopyrene-eupyrene sperm males produced. In experimental and natural populations, the number of oligopyrene sperm produced, but not the number of eupyrene sperm, was significantly higher when the sex ratio was male biased. Testis mass did not vary between experimental treatments. We also found a negative relationship between the number and size of oligopyrene sperm produced, which is consistent with evolutionary models of sperm competition, and is, to our knowledge, the first intraspecific demonstration of a trade-off between these traits.