Genes that are directly involved in reproductive success are under intense selective pressure, and life at the sharp end of evolution is no more readily observed than in the case of sperm competition. That such competition occurs in man is controversial, but sperm wars are particularly well documented in Drosophila. Female drosopholids store sperm from successive mates, and the last male to inseminate fathers most of the offspring. The obvious explanation is that the organisation of the female’s sperm stores means simply ‘last in, first out’. Given the prize at stake, something more devious than a gentlemanly, ‘after-you-old-chap’ arrangement was always suspected, but hard to prove. Some researchers, although not all, found that new seminal fluid can disable sperm from previous inseminations. To re-examine this phenomenon, Price et al.1xSperm competition between Drosophila males involves both displacement and incapacitation. Price, C.S.C. et al. Nature. 1999; 400: 449–452Crossref | PubMed | Scopus (122)See all References1 used spermless mutant males (XO males) to inseminate females two or seven days after a previous mating, and found that the spermless seminal fluid reduced fertilization by seven day-old sperm – but not as much as a normal ejaculate – and had no effect on two-day-old sperm. The seminal fluid seems to block or incapacitate the seven-day-old sperm because the latter does not disappear. Next, to look at normal ejaculate, Price et al.1xSperm competition between Drosophila males involves both displacement and incapacitation. Price, C.S.C. et al. Nature. 1999; 400: 449–452Crossref | PubMed | Scopus (122)See all References1 produced a strain of flies that express green fluorescent protein in their sperm tails and watched how these sperm went about jumping the queue. Seemingly, the first male’s sperm are physically displaced, though not through lack of space. How precisely the mechanisms of displacement and incapacitation work is a mystery, as is the role played by the female manipulating or selecting sperm. No doubt, though, there is a vigorous genetic arms race going on underneath.