Fighting by organisms such as mice and Drosophila provides model systems for investigating the genetic basis of aggression. Recent experiments to dissect male aggressive behaviour in Drosophila melanogaster, using gene expression analysis of selected lines followed by mutant analysis, have identified new candidate genes associated with male aggression, including one strong candidate that encodes a cytochrome P450 enzyme. Here, we describe the study of aggressive behaviour in flies and explore the possibility that cytochrome P450 is involved in aggression.