We examined the effects of male and female behaviour and morphology on the process of sperm storage in the yellow dung fly Scathophaga (Scatophaga) stercoraria. The larger of two males was more successful in transferring sperm to females' spermathecae the greater the difference in weight to his smaller competitor, as expected from previous studies by other authors. Sperm length, which is not correlated to body size, affected sperm access to the spermathecae, the female storage organs; longer sperm were more likely to be found in the spermathecae. A female typically had a singlet spermatheca and two spermathecae arranged as a pair, a doublet. However, there was variation from this pattern, which influenced the pattern of sperm storage. We measured the proportion of sperm from two competing males in females' singlet and doublet spermathecae. When the larger male's sperm were longer than his competitor's, they were more often in a female's singlet when he was her first mate and equally likely to be in the singlet or doublet when he was her second mate. When the larger male's sperm were shorter than his competitor's, the pattern was more complicated, principally because his sperm were not as successful at entering the female's doublet when he was her second mate. Counts of sperm, made using the same experimental procedure, showed that these effects were due to greater numbers of sperm entering the females' doublets when the larger male mated second. Sperm length was thus the factor with the largest single influence on the pattern of sperm storage. However, our most important result is that it was the interactions between male and female characters that were significant. Males mostly determine the early, especially precopulatory, events and females strongly influence the later ones.