Aggression is nearly ubiquitous throughout the animal kingdom, and plays a major role in determining individual fitness and the evolutionary trajectory of a species. Although most scientific research has focussed on male aggression, recent work has begun to reveal that aggression by females towards other females is common amongst animals, and can have important evolutionary consequences. In the fruit fly, Drosophila melanogaster, both sexes engage in same-sex aggressive behaviour, but historically research has been heavily biased towards males. However, male and female intrasexual aggression may differ substantially in contest dynamics, level of escalation, and reasons to display aggressive behaviour. One avenue in which sex differences could affect intrasexual aggression is that of mating. Males and females differ in their responses to mating – female behaviour and physiology change dramatically after mating in a wide range of taxa, often mediated by substances transferred in the male ejaculate. We investigated the effects of mating on female aggression in D. melanogaster, and found that mating dramatically increases female aggressiveness: it induces females to approximately double their time spent in aggressive encounters. We also found evidence that the seminal fluid protein “sex peptide”, which mediates several post mating responses, including increased egg-laying and decreased receptivity, may play a role in increasing female post-mating aggression. We also tested the effects of egg production on female post-mating aggressiveness, as theory predicts that individuals with higher resource demands (i.e. mated females producing eggs need high levels of protein) should be more aggressive than those with lower resource demands (virgin or mated sterile females). However, our results suggest that feeding, fecundity and aggression can be uncoupled. These results suggest that females are responding directly to male mating signals in their interactions with other females, opening up possibilities for sexual conflict or male manipulation of female-female interactions.