Communication in the natural environment often involves more than a simple sender-receiver dyad because signals may be detected by more than one individual (i.e. communication occurs in networks). The presence of individuals other than those involved in the signalling interaction has been shown to change signallers' behaviour. Previous experiments have shown that intra-sexual communication of male fighting fish (Betta splendens) is affected by the presence of a female but not by a male conspecific. However the experimental design did not allow the effect of the sex of the audience to be compared. We used an experimental design that allowed direct investigation of the effect of the sex of an audience on male-male fighting fish interactions. Our results show that the sex of a conspecific audience influences male-male aggressive displays. When a male audience was present subjects attempted significantly more bites and spent less time near the opponent than with a female audience. The results of this experiment support the view that the presence and sex of an audience is important in determining how individuals should display during an interaction.