Stomach and small intestine development was characterized in tammar wallaby (Macropus eugenii) pouch young (PY) using both morphological and immunohistological criteria. At birth, the stomach is undeveloped in comparison to the well‐developed intestinal mucosa. The stomach maintains a uniform morphology in both the forestomach and hindstomach regions until the specialization of cardiac and gastric glands are seen at PY170. Parietal cells, found throughout the mucosa are downregulated in the forestomach as cardiac glandular stomach is developing prior to the transition of the offspring to a diet that includes herbage. In the small intestine, mature‐type villi are present at birth but the muscularis externa is immature and undergoes significant development around PY120 onwards. We investigated the effects of changes in maternal milk on gut development in the tammar wallaby using a cross fostering approach that provided younger pouch young with older stage milk. Younger PY (average age 67 days postpartum, n=5) were transferred onto teats vacated by older stage PY (average age 100 days postpartum, n=6) for 34 days before gut development was assessed. In addition milk analysis was performed before and after fostering events. Cross‐fostered PY animals receiving older stage milk were found to be 31% heavier than controls. There was no difference between carbohydrate and protein concentrations however, fostered PY milk had a higher concentration of lipid than that of controls that may have contributed to heavier fostered PY. No difference was found in stomach or small intestine development between these groups using the criteria employed in this study. J. Exp. Zool. 303A:331–344, 2005. © 2005 Wiley‐Liss, Inc.