Sexual attractiveness of calypterate Diptera may be measured by their ability to elicit encounters from individuals of opposite sex. Stationary female S. stercoraria elicit a much higher encounter rate from searching males than do stationary males and immobile pairs elicit an intermediate rate. It is therefore concluded that males can 'recognise' these three types of individual; probably by visual means. Sexual attractiveness after contact may be measured by the extent to which a contact courtship bout progresses or by measuring the 'bout duration'. There is no difference between the bout durations of males with other males and that with pairs. This implies that on contact there is no ability to discriminate between pairs and males. This is not true, however, when an attacking male manages to touch a paired female during a contact bout. This results in a struggle between males for the possession of the female. Paired males perform characteristic rejection response patterns to different types of encounter bout, depending on the direction of attack and the extent to which the bout proceeds. The responses appear to have evolved to prevent touching of the female by the attacker. By far the most common type of bout occurs when the attacker mounts a pair. The paired male almost always reponds by raising both middle legs and 'standing' (straightening the front legs), a reaction which doubles the distance between the attacker and the female and thus effectively prevents contact of the attacker with the female. This is essentially a contact reaction, since a premature response would allow direct access to the female rather than having the effect of 'shrugging off' the attacker. The rejection reactions of paired males could have arisen from the avoidance reactions of single males. Single ('searching') males perform most of the reactions of paired males but less vigorously, and in addition perform swaying movements similar to those of females. The avoidance reactions of single males may have adaptive value in reducing the time wasted in a bout with another male. With both single and paired males, the intensity of the rejection response elicited is proportional to the extent to which the bout proceeds after contact. Males in the passive phase (i.e. paired to ovipositing females) react most intensely to a given type of bout. Struggles occur in about 7% of encounters with both type of pair and result from failure of the rejection responses, instability, or multiple attacks. They involve up to several minutes time waste before one male finally dominates. Take-overs (where the original male is ousted) are much more frequent in attacks of pairs involved in oviposition than with pairs in genital contact, occurring in 1.75% and 0.65% of encounters respectively. The second male then begins genital contact immediately.