Five species of large venomous snakes of the genus Pseudechis are oviparous, whereas one (Pseudechis porphyriacus) is viviparous. Data were gathered on body sizes, food habits, activity patterns and reproductive biology of all the Pseudechis species, based on dissection of 996 museum specimens and maintenance of animals in captivity. These data revealed that oviparous and viviparous congeners do not differ in most of the factors predicted to favor the evolution of viviparity, or to evolve in response to viviparity. Specifically, all of the Pseudechis species are similar in average adult body size (100-130 cm), sexual size dimorphism (adult males approx. 15% longer than females), seasonal timing of reproduction (ovulation in spring, at least in the temperate zone), fecundity (averages of 8-12 offspring), and sizes of eggs and offspring (hatchlings of 200-300 mm snout-vent length [SVL] except in P. colletti, 280-370 mm SVL). Interspecific differences were evident in activity patterns (diurnal to nocturnal) and food habits (catholic, eating mainly reptiles, frogs and mammals), but geographic variation in these characteristics in wide-ranging species suggests that such differences reflect phenotypic plasticity rather than fixed interspecific differences. The only unique characteristic of the viviparous species is its occupancy of cold climates, and possibly its restriction to riparian habitats. These data strongly falsify several hypotheses on morphological and ecological consequences of the evolution of viviparity. Hypotheses on selective forces for the origin of viviparity are much more difficult to test. Of the many alternative hypotheses on this question, the "cold climate" hypothesis is the only one which successfully predicts ecological correlates of reproductive mode in Pseudechis.