An allometric analysis of reptilian incubation time was performed. Incubation time increased with increasing egg mass (allometric exponent = 0.14, r2= 0.31). However, significant heterogeneity was found within the Reptilia for this character. If periods of developmental arrest are excluded the orders Crocodylia, Testudinata, and the suborder Serpentes have the shortest incubation times and the lizards the longest. Significant differences within lizard families were also observed (Iguanidae, Agamidae, Gekkonidae, and Lacertidae have shorter incubation times than the Varanidae and Teiidae). Preliminary analyses indicate the heterogeneity in incubation time observed within the Squamata is correlated with differences in brain mass; this is similar to findings for mammals. The variation in incubation time indicates the existence of differences in growth rates in reptilian groups and suggests a partial explanation for the observed distribution of viviparity within the Squamata.