In Australia, there are around 60 species of murid rodents that occur in the subfamily Hydromyinae, most of which produce highly complex, monomorphic, spermatozoa in which the head has an apical hook together with two ventral processes containing filamentous actin and a long tail of species-specific length. One of the few exceptions to this is the spinifex hopping mouse, Notomys alexis, whose spermatozoa have previously been shown to have pleiomorphic heads. In this study, the structural organisation of the sperm head has been investigated in more detail and the variability in length of the midpiece and total length of the sperm tail has been determined for this species. The findings confirm that pleiomorphic sperm heads are invariably present in these animals and that this variability is associated with that of the nucleus, although nuclear vacuoles were not evident. The total length of the sperm tail, as well as that of the midpiece, was also highly variable both within, as well as between, individual animals. The reason(s) for this high degree of variability in sperm morphology is not known but it may relate to a relaxation of the genetic control of sperm form owing to depressed levels of inter-male sperm competition.