In mammals spermatozoa show great morphological differences across species.  Most murid rodents have sperm heads with an apical hook whereas most Australian Old Endemic murine rodents (Tribe Hydromyini) have sperm heads with two further cytoskeletal structures, the ventral processes, that extend from the upper concave surface.  Two hypotheses have been proposed to explain interspecific differences of sperm size and shape - (1) morphology of the female reproductive tract, and (2) intensity of intermale sperm competition.  In this study we test the latter hypothesis using material from 24 species of Australian Old Endemic rodents.  We compared sperm head area, maximum head length and breadth, apical hook and ventral process length and orientation, as well as midpiece and total tail length to the relative testes mass (RTM) which is used as a proxy for intensity of intermale sperm competition.  Our observations show that RTM ranges from 0.1 to 3.5 percent of body mass across the species, suggesting marked interspecific differences in sperm competition. The initial regression analyses on our preliminary observations show that head area has a significant negative linear relationship with RTM (p < 0.01), whereas maximum head length, hook angle and ventral process length all have a significant positive linear relationship with RTM (p < 0.05).  The other traits measured have no significant linear relationship with RTM (p > 0.05).  These preliminary findings support the conclusions of some, but not all, previous studies most of which have used other species groups to investigate the effects of differing intensities of intermale sperm competition on sperm design.  The present results suggest that, within this monophyletic group of Australian rodents, sexual selection has been a major driving force in reducing overall sperm head area, maximising head length, increasing curvature of the apical hook and increasing the length of the ventral processes.