This thesis presents experiments which investigated the genetics and mechanisms of sperm displacement in the fly Drosophila melanogaster. Sperm displacement allows males to increase their reproductive success relative to other males. Differential reproductive success of males is an important aspect of total fitness. D. melanogaster provides an effective system in which to investigate traits that affect reproductive success. The experimental work presented involved both genetic studies and phenotypic manipulations. Measurements of the repeatability and heritability of sperm displacement ability (SDA) showed both parameters to be low but significantly nonzero. SDA was correlated with pre-adult viability, suggesting the heritable component of SDA and pre-adult viability may be confounded. Since little is know of the mechanism of sperm displacement in D. melanogaster, experiments were carried out to investigate the effect of variation in factors which may possibly influence SDA. It was determined that males did not displace the sperm of other males in preference to their own sperm. The effect of variation in copulation duration was investigated with both spermless and normal males. The results indicated that levels of sperm displacement were determined by the amount of sperm transferred, a process which appeared to occur rapidly during copulation. The effects of variation in genetically determined male body size was also investigated. Larger males were found to be at a slight disadvantage in virgin-female matings. No significant effect of body size on SDA was observed. Separate experiments were carried out to determine the effect of male accessory gland products on the fertility of subsequent matings. The possibility that "sperm defence" is due to accessory gland fluid physically interfering with the storage of second male sperm was shown to be unlikely.