We investigated the dynamics of sperm transfer and the potential conflict between sexes over mating opportunities in the bumblebee, Bombus terrestris L. We recorded copulation duration in flight cages and manipulated copulation time by separating mating pairs. Sperm transfer and filling of the spermatheca were measured by dissecting queens at different time intervals after the onset of mating. On average, copulations lasted 37 min but most sperm were transferred into the female genital tract within the first 2 min. Sperm reached the spermatheca after 30-80 min. Males transferred a sticky gelatinous product of their accessory glands, the 'mating plug', to the female within 10-30 min of the onset of copulation. Hence the duration of the copulation matched the time required to deposit the plug fully. A likely function of the plug is to prevent backflow of sperm, but by artificially transferring mating plugs we showed that the plug also decreased sperm transfer into the female's spermatheca for at least 4 h and possibly for much longer (8 h or more). Increased mating costs arising from male selfishness may prevent females from seeking further matings. This may help to explain why females of B. terrestris seem to be mostly singly mated in the field, even though, in experiments with artificially inseminated queens, higher sperm diversity increases reproductive output. This is the first report of a mating plug for the Bombini but similar devices are known from two other tribes of the Apidae, the Apini and Meliponini. Copyright 1999 The Association for the Study of Animal Behaviour.