A twenty-week experimental feeding period was conducted on Oreochromis niloticus fry to study the effect of different dietary protein levels and Oxytetracycline on the bacterial count inhabiting intestine of Nile tilapia fingerlings (Oreochromis niloticus) . Two protein levels (25 and 40%) were used within each protein level 4 oxytetracycline levels (0, 50, 100 and 150 mgjkg diet) were added.One thousand- six hundred fry with average weight of 0.52 g were randomly distributed to 16 aquaria (160 L each), each treatment was performed at two replicates. The addition of the antibiotics to the diet enhanced growth when the diet contained a high level of protein (40%). The predominant species of bacteria inhabiting the anterior, middle and posterior part of the intestinal tract of Oreochroumes niloticus were affected by the protein and oxytetracycline levels. The highest bacterial number was obtained in the posterior part of the alimentary canal of fish. It is suggested that this drug has a sparing effect on the dietary protein normally used for energy, and may possibly be used (under controlled conditions) as growth enhancer. Withdrawal periods of oxytetracycline in a treated fish were 22 days.