The antibacterial effect of oxytetracycline (OTC) on bacterial load, pathogen and possible development of antibiotic resistance in intestinal flora of apparently healthy Nile tilapia (Oreochromis niloticus) was investigated under laboratory condition. OTC, a broad spectrum approved antibiotic widely used in aquaculture, was fed to Nile tilapia through medicated diet at a rate of 2 g/ kg of feed for consecutive 7 days in treatment 1 (T-1) and 30 days in treatment 2 (T-2). Changes in physico-chemical parameters of water were also recorded where pH and dissolved oxygen influenced bacterial load and coliform count respectively. At the start of the experiment, initial bacterial load in fish intestine was 1.61 +/- 2.25 x 10(10) cfu/g, which after OTC treatment decreased significantly (p <.05) to 3.06 +/- 2.08 x 10(8) cfu/g on day 7 and 3.45 +/- 4.46 x 10(7) cfu/g on day 30 when compared with the control group. Among the 10 bacterial genera identified in the intestine, 4 potential public health concern bacteria viz., Salmonella, Escherichia, Enterobacter and Staphylococcaus were predominant. There was a sharp increase in bacterial load on day 18 in T-2 that may be attributed to the development of antibiotic resistance in intestinal bacteria. Antibiotic susceptibility test for isolates against seven antibiotics: amoxycillin, ciprofloxacin, co-trimoxazole, erythromycin, gentamicin, nitrofurantoin and tetracycline using disc diffusion method revealed significantly increased resistance of Gram-negative rods (p <.05) that possibly caused increased frequency of OTC-resistant microorganisms. In conclusion, short- and long-term exposure to OTC treatment affected the distribution of bacterial genera including pathogens in the Nile tilapia gastrointestinal tract and concomitantly influenced their antimicrobial resistance.