Sperm form and size is tremendously variable within and across species. However, a general explanation for this variation is lacking. It has been suggested that sperm size may influence sperm competition, and there is evidence for this in some taxa but not others. In addition to normal fertilizing sperm, a number of molluscs and insects produce nonfertile sperm that are also extremely morphologically variable, and distinct from fertilizing forms. There is evidence that nonfertile sperm play an indirect role in sperm competition by decreasing female remating propensity in Lepidopterans, but in most taxa the function of parasperm is unknown. We investigated the role of nonfertile (oligopyrene) sperm during sperm competition in the fresh water snail Viviparus ater . Previous studies found that the proportion of oligopyrene sperm increased with the risk of sperm competition, and hence it seems likely that these sperm influence fertilization success during competitive matings. In mating experiments in which females were sequentially housed with males, we examined a range of male characteristics which potentially influence fertilization success. We found that the size of oligopyrene sperm was the best predictor of fertilization success, with males having the longer sperm siring the highest proportion of offspring. Furthermore, we found a positive shell size and sperm concentration effect on paternity, and females with multiply sired families produced more offspring than females mating with only one male. This result suggests polyandry is beneficial for female snails.