Marked interspecific differences in the morphology of the male reproductive tract of Australian hydromyine rodents have been found. In males of most species, the structure of the vas and proximal accessory sex glands was similar to that in other murids, but most Notomys had a distal vas with an unusually large lumen, which may act as a sperm store, and very large ventral prostates but reduced seminal vesicles, dorsolateral prostates, coagulating and ampullary glands. Bulbourethral glands showed little interspecific variation, but preputial glands were large in Pseudomys and Leggadina, minute in Notomys, and apparently absent in Hydromys, Melomys and Uromys males. The erect glans penis morphology was generally similar to that of other murids, except that in most Notomys there was a narrow shaft with unusually large spines, no large cup, and slender baculum. The functional and evolutionary implications of the findings are discussed.