Success in sperm competition is one of the principal determinants of male fitness in species in which females mate promiscuously, but the selective pressures it causes are only partly understood, especially with respect to sperm characteristics favoured under sperm competition. Corelates of male success in sperm competition were examined in the bulb mite (Rhizoglyphus robini). The effects of the characteristics of individual male's ejaculates (sperm size and number), their body size and copulation duration were examined. Only sperm size was significantly correlated with sperm competition success: males producing larger sperm were more successful. Moreover, ejaculates of males producing larger sperm also had significantly smaller variation in sperm size, which indicates that they were able not only to allocate more resources to sperm but also to maintain more stable allocation per gamete. There was no significant correlation between sperm size and number of sperm per ejaculate.