Among nocturnal Malagasy prosimians, the grey mouse lemur (Microcebus murinus) is considered as a solitary species having a promiscuous mating system. Indirect evidence, such as the lack of sexual dimorphism, the high relative testes size of males and the high synchronism in oestrus occurrence among females, supports the presence of a sperm-based scramble competition. In captive animals, we used genetic determination of paternity to define the relationship between mating success and reproductive success of males kept in a group with several females during the mating period. Within each group, an intense sexual competition arose among males for priority of access to oestrous females. High ranking males, as deduced from the direction of agonistic interactions, displayed a significantly higher frequency of marking behaviours, chemosensory investigations of females, and sexual behaviours. A robust relationship was found between male rank and reproductive success since 16 out of the 17 litters (35 young) produced, have been sired by the highest ranked male of the group. Moreover, the finding that female aggression was related to both male sexual solicitations and to male rank, suggests that females could exert a choice. Under specific captive conditions, grey mouse lemurs, showing all traits of post-copulatory sperm-based scramble competition, exhibit a mating system relying on pre-copulatory aggression-based competition. Olfactory signals rather than morphological traits might be used, in this nocturnal primate, for both sexual co-ordination between sexes and partner choice.