Ancient roles of CRF and ACTH in immune–endocrine interactions were obtained early in the development process of fish alimentary canal. The immunolocalization of corticotrophin releasing factor (CRF) and adrenocorticotropic hormone (ACTH) was inspected in the developing gut of Oreochromis niloticus larvae. The aim was to investigate a possible involvement of these molecules early in the integration of immunological and endocrine systems. Immediately after hatching, the gut of O. niloticus is observed as a straight undifferentiated tube, and with the rapid development, it differentiates into four segments: buccopharinx, esophagus, presumptive stomach and intestine. The immunohistochemical investigation showed the immunolocalization of CRF in the growing digestive tract at all stages (from hatching to 42 days post-hatching). Immunoreaction of CRF was detected in the mucosal epithelium of both the undifferentiated gut and the developing esophagus, stomach, and intestine. Furthermore, CRF immunoreactivity was found in the gastric glands of the stomach. The number of CRF-immunoreactive (ir) cells and the strength of immunoreaction gradually increased as the larvae developed, particularly after the exogenous feeding began; 21 days after hatching. Only the goblet cells of the developing intestine exhibited an ACTH immunoreactivity, which increased at 7dph during the yolk sac resorbtion period. A dramatic decrease was recoded in the number and size of ACTH-ir cells  associated with the beginning of the exogenous feeding, and at 28 days post hatching, a very weak immunoreaction was produced. The widespread anatomic distribution and early onset of CRF and ACTH activities, in the developing gut, indicate that these molecules play a functional role in food intake, growth, immunological response, and osmoregulation during O. niloticus development, particularly with the start of the exogenous feeding.