Post-natal growth of Rattus exulans was observed in 30 litters born from wild-caught females. Detailed observations of behavioural development were made on 10 litters. The physical development of the young is described and weight and length increases analyzed. Litter size varied from two to six with a mean of 3.6 ± 0.21. Major behavioural changes correlated closely with patterns of physical development. These changes occurred at 5, 15 and 22 days, and divide the post-natal development into the neonatal (0–4 days), transitional 5–14 days), socialization (15–22 days) and juvenile periods (23 days to sexual maturity). The patterns of development in Rattus exulans were considered conservative, not specialized, which is of obvious advantage to a commensal rodent and account, in part, for the success of the species as a colonizer of islands over a vast area of the Pacific.