Sperm displacement has been the subject of a large number of evolutionary studies because of its effects on relative male reproductive success. To understand better the evolutionary role of variation in sperm displacement ability (SDA), an obvious aim is to measure its heritability. In this paper, we show that a standard method used to measure the heritability of SDA can be misleading. First, we show that using conventional methods (based on counts of adult offspring of multiply mated females), SDA appears to be heritable. However, an examination of potentially confounding variables strongly suggests that this result is misleading, and that the heritable component is more likely to be pre-adult viability. Consequently, it is likely that there is little measurable heritable genetic variation for SDA in D. melanogaster. We conclude that, although conventional methods of measuring sperm displacement will usually be adequate for phenotypic measurements, greater care must be taken when measuring genetic variances.