Unlike the remainder of the rodent subfamily Muridae, Acomys cahirinus (the 'spiny' mouse) is born in a precocial state after a long (39 day) gestation. In this paper, the development of the olfactory bulb, neocortex and hippocampal formation of Acomys from prenatal days 14-34 was examined and the rate of maturation compared with that of its cousin, the laboratory rat (Rattus norvegicus). At the earliest stages examined, Acomys was approximately 2 days less mature than the same post-conception aged rat. The difference between the two species increased: Acomys at 28 days postconception resembled the 22-day rat. By the end of gestation, Acomys and the rat were in a relatively similar developmental state. Therefore, Acomys exhibits a quite different timetable of early maturation which includes a protracted period of relatively slow growth during mid-gestation. As such, it offers many benefits as a subject for studies of both early ontogenesis and the mechanisms which result in species differences.