Why are viviparous squamate reptiles more common in cold climates, and oviparous ones in warmer areas? The usual explanation is that (1) oviparous squamates cannot reproduce successfully in cold areas because soil temperatures are too low for embryonic development; and (2) viviparous squamates experience lower survivorship or reproductive success than oviparous taxa in warmer areas. These hypotheses suggest that the boundaries of geographic distributions of congeneric oviparous and viviparous squamates should be predictable from data on thermal tolerances of embryos, and estimated temperatures of soils and gravid female reptiles throughout the potential geographic range of the taxon. In large venomous Australian snakes of the genus Pseudechis, distributional boundaries of oviparous and viviparous taxa can be accurately predicted from such data. This predictive ability, if substantiated by studies of other reproductively biomodal squamate taxa, would support the putative role of reproductive mode as a direct determinant of reptilian geographic distributions.