Arid zone old endemic Australian rodents include the spinifex hopping mouse (Notomys alexis) and the plains mouse (Pseudomys australis) both of which undergo “boom” and “bust” population cycles. How similar is their reproductive biology? Laboratory studies have shown that, compared to most murine rodents, both species have low reproductive rates with pregnancy length in non-suckling females being 30-36 days, litter size around 3.5, and maturation at 50-60 days. Dramatic differences are nevertheless apparent in their male reproductive tract morphology and mating behaviour. Mature plains mice have a large relative testes mass (RTM) of 2-4% body mass (mean body mass 55-60 g; testes mass ca. 2,000 mg), large epididymides with number of stored sperm ca. 500x10⁶, and very large seminal vesicles and coagulation glands. By contrast, sexually mature hopping mice have maximum RTM of only ca. 0.15% (mean body mass 30-37 g, testes mass ca. 40 mg), very small epididymides with only ca. 1x10⁶ sperm, minute seminal vesicles and coagulation glands but larger ventral prostates. Mating behaviour in hopping mice, but not plains mice, involves locking for up to several minutes. Related to this male hopping mice have a penis with much larger spines but narrower shaft and smaller baculum, and females a more muscular vagina with narrower lumen and much less fibrous, smaller, cervix. The huge differences in RTM suggest marked interspecific differences in mating system with hopping mice exhibiting monogamy and plains mice polyandry/promiscuity. Preliminary laboratory data support this conclusion; field observations are now required to test this hypothesis.