Sperm competition theory predicts that males should allocate sperm prudently in response to perceived return on investment. It is predicted that males should invest in greater sperm stores and allocate more sperm to females in response to increased risk of sperm competition (absence versus presence of competitors), but allocate fewer sperm when intensity of sperm competition (number of competitors) is high, owing to diminishing returns. We tested these predictions in Gammarus duebeni, an amphipod with precopula mate guarding. Contrary to predictions, we found no difference in sperm allocation to females in response to increased risk or intensity of sperm competition. In addition, we observed a decrease in both sperm numbers and sperm length when males were kept with potential competitors for extended periods of time, particularly in larger males. We suggest that G. duebeni invests in mate guarding over sperm production/allocation in response to sperm competition, and we discuss whether competition for resources may also affect investment in ejaculate production.