In this study, the stomach of Oreochromis niloticus fish was examined during their growth from the time of hatching to the adult stage using light and TEM. At the time of hatching the digestive tract was represented by undifferentiated tube located dorsal to the yolk sac. Differentiation of the digestive tube into oesophagus, presumptive stomach and intestine by the second day posthatching. In addition to the appearance of dense secretory granules in the first part of the stomach (cardiac) lining epithelium that reacted only to PAS. Mucous glands were also detected at the cardiac by the day seven after hatching. These glands reacted positively with PAS and alcian blue stain. As the larva grew, acidophilic glands lined by cuboidal cells reacted negatively for both PAS and alcian blue started to appear in the second part of stomach (fundic) at nine days posthatching. Fine structure of these glands revealed large basal euchromatic nucleus, free ribosomes, dilated rER., well developed Golgi, complex system of tubules and vesicles and the numerous large mitochondria with tubular cristae. By the ten days after hatching, mucous glands began to appear in the third part of the stomach (pyloric). At the ultrastructure level, the glandular cells were joined to each other by multiple desmosomes. Their cytoplasm was mostly occupied by electron dense granules of different sizes and densities. By 14 days after hatching the stomach was completely formed.  Physiological implications of some ultrastructure features were also discussed