In eutherian mammals, sperms passing through the epididymis undergo subtle morphological and biochemical changes which culminate in the capacity to fertilise. In contrast, the pattern of sperm maturation in the possum is extremely elaborate and overt. The spermatozoa undergo marked contraction and loss of cytoplasm. Structural elements are elaborated in the midpiece and they progressively acquire the potential for motility. Unlike the eutherian, these changes are easily studied using light microscopy. As well as being an inherently interesting picture of cell differentiation, sperm maturation in the possum also provides a useful model for looking at the relative importance of testis versus epididymis in the evolution of male reproduction. Far from being a 'primitive' system, sperm maturation in the possum epididymis appears to be more elaborate and specialised than in comparable Eutheria. These findings are discussed in the light of experiments to determine the endocrine control of epididymal function in the possum.