Among all species in the genus Drosophila whose sexual behavior has been studied, D. pegasa is unique in that males exhibit no courtship behaviour before they mount females. Instead, the male simply climbs on top of the female and rides on her abdomen ('grasping') for long intervals, often an entire 8-h observation period. In this study we conducted a series of observations and experiments to quantitatively describe grasping behaviour in D. pegasa and examine its relationship to social environment and the reproductive biology of the species. All observed courtship bouts involved grasping behavior, and males always initiated copulation during a grasping bout. The frequency of grasping and the average duration of a single grasping bout increased with the number of flies present. Males often copulated several times during a single grasping bout, and such multiple copulations during a single mounting also appear unique in the genus. Unexpectedly, the number of sperm that a male transferred to a female during a single grasping bout was negatively correlated with the number of copulations. This relationship was apparently due to repeated copulations by males who were unsuccessful at transferring sperm. Multiple copulations without sperm transfer may result from cryptic female choice. Male grasping behaviour in this species appears to have evolved as a substitute for display and courtship behaviours, but possibly also as a mate-guarding behaviour since males continue to grasp after they have successfully transferred sperm. The tarsal claws and pulvillar pads of D. pegasa are disproportionately larger than those of related Drosophila species, evolutionary changes that may facilitate grasping by males.