The number, distribution, maturation, motility and ultrastructure of spermatozoa from both northern (Isoodon macrourus) and southern (Isoodon obesulus) brown bandicoots were examined. One epididymidis per animal was fixed for light microscopy and transmission electron microscopy, and the contralateral side was used for the determination of sperm number, distribution and motility. Sperm form was similar between the two species. Approximately 56 x 10(6) testicular sperm and 100 x 10(6) epididymal sperm per side were present in I. macrourus, about 60% of which were in the caudal region. Initiation of sperm nuclear rotation and loss of the cytoplasmic droplet was first observed in distal caput or proximal corpus segments along with slow progressive motility. In these sperm, dislocation and anterior movement of the sperm neck from the implantation fossa and the modification of the distal margins of the sperm acrosome were evident. Motility of cauda epididymidal spermatozoa was rapid and coordinated, movement was restricted to one plane, and lateral head displacement was marked. As media viscosity increased, sperm velocity decreased, as did the amplitude of the tail beat, its frequency, and lateral head displacement but, in viscous mineral oil and mixtures of media and prostatic exudate, extremely rapid sinusoidal motility occurred. This study has detailed unusual morphological changes in bandicoot sperm during epididymal maturation and has shown that, although bandicoot sperm differ morphologically from those of the dasyurids, particularly in relation to head-tail orientation and tail ultrastructure, they exhibit similar motility.