A field investigation was made of the reproductive behaviour of Sepsis cynipsea (L.) around cattle droppings. Most of the reproductive activities occur within the first hour of deposition of the dropping and males arrive in often immense numbers so that there is intense intra-male competition for females. Males capturing females on the dropping do not attempt to copulate, but adopt a passive phase (where the male remains mounted but without genital contact) while the female oviposits. Calculations from curves showing the incidence in relation to dropping age of individuals in various behaviour stages indicate that males would probably achieve approximately the same capture rate of females irrespective of the actual time they spend on the dropping (the 'stay duration'). This situation is explicable in terms of equilibration of proportions of males with the different stay durations, through sexual selection, so that all males experience the same female capture rate. The stable proportions should be determined by the rate of arrival of females and the time taken to transfer from one dropping to another. Searching males are found in greatest density in the I0 cm of vegetation immediately peripheral to the dropping edge, though the density on the dropping is also high. This may be related to the aggressive behaviour and possible territoriality of males on the dropping surface. Some 40% of gravid females arriving at the dropping begin to reject their males immediately after establishment of the passive phase; this may occur because of capture in the surrounding grass rather than on the dropping. Females on the dung are all involved in oviposition; when all the mature ova are laid the females emigrate from the dropping for some distance into the surrounding grass. During emigration, the behaviour of the passive phase male undergoes a very marked change. 'Copulation precursors' (strokes of the female abdomen by the male genitalia, and temporary genital contacts) show a drastic increase in frequency of occurrence. In females which eventually copulate, rejection tendencies (which become apparent early in emigration) gradually subside and temporary genital contacts increase in duration and frequency, so that immediately before copulation (which occurs on average some 7 min. after leaving the dung) about 80% of the time is spent in temporary genital contact. True copulation takes some 23 min. and terminates with a specialised 'separation reaction' in which the male, at first pivoting at the coupled genitalia, swings around in 2-3 full circles in either clockwise or anticlockwise directions. Some 40% of females do not eventually copulate after emigration, instead the female rejection components (mainly a vigorous lateral swaying reaction) increase and become persistent, suppressing the male copulation precursors, which begin as normal when the female leaves the dropping. Rejection takes on average about 6 min. and the pair separate without a separation reaction. Wing and abdomen waving responses are conspicuous in Sepsid behaviour and are especially important in the male reproductive behaviour of S. cynipsea. They are prevalent in encounters and locomotory activity, and are elicited in a passive phase male when an ovipositing pair is attacked by a single male. The rate at which encounters from searching males are sustained is higher on the dung surface than in the surrounding grass. 'Take-overs' (a change over in male in possession of the female after an encounter) are very rare, apparently occurring in less than 0.2% of encounters. 'Groups of 3' (in which a second male is mounted on the female at the side of the first male) also occur infrequently, and can lead to 'take-over'. Oviposition behaviour is analysed. S. punctum has reproductive behaviour which is essentially similar to S. cynipsea, except that the 'take-over' frequency is mugh higher and copulation apparently begins immediately on emigration after oviposition. The present knowledge of reproductive behaviour in other Sepsids is reviewed. Post-oviposition copulation is a very rare phenomenon. How and when virgin females of S. cynipsea are mated is not known.