Although viviparity (live-bearing) has evolved from oviparity (egg-laying) more frequently in squamate reptiles than in any other vertebrate lineage, there are few well-documented cases of taxa that either (i) exhibit a 'transitional' reproductive state (i.e. with a reproductive mode intermediate between 'normal' oviparity and viviparity) or (ii) contain both oviparous and viviparous populations within the same species. Although rare, such taxa offer exceptional opportunities to test hypotheses concerning the evolution of viviparity in reptiles. Our data show that the scincid lizard Saiphos equalis displays both of the characteristics listed above. These small semi-fossorial skinks from south-eastern Australia exhibit geographic variation in reproductive mode, and some populations show an 'intermediate' mode. We examined the reproductive mode of Saiphos equalis over the geographic range of the species using preserved museum specimens, and we gathered detailed information on reproductive output of captive lizards collected from a high-elevation site (Riamukka, in the northern highlands of New South Wales) and from a coastal area (Sydney, southern New South Wales). Lizards from Riamukka were viviparous (i.e. they produced fully formed young enclosed in membranous sacs), whereas Sydney lizards produced incompletely developed embryos inside partially calcified eggshells. Incubation periods of the eggs from Sydney lizards were very brief (5.5 ± 1.7 days v. >35 days in sympatric oviparous skinks), indicating that oviparous S. equalis represent a true evolutionary intermediate between 'normal' oviparity and viviparity.