The mating behaviour of Bombus terrestris was studied using wooden observation cages (50 x 50 x 70 cm) in the laboratory. Queens and males from reared colonies were isolated from the odour of the opposite sex before the experiments. Mating pairs (one per box) consisted of one 10-day-old virgin male and one 5-day-old virgin queen. The behaviour of each of the 80 pairs used was observed for 15 min, during which time 47 of the pairs copulated. Male behaviour could be divided into approach, inspection and attempt to copulate. Queen reactions to the male were immobility (13.8%), threat (53.9%) or flight (32.3%). Queens could also display inspection behaviour. The mating success of a behavioural sequence was positively correlated with the frequency of inspections by both sexes and negatively correlated with queen immobility. Flow diagrams showing sequences of types of behaviour are presented. The role of sexual pheromones in mating is discussed.