Oxytetracycline (OTC) residue in Nile tilapia Tilapia niloticus muscle was assayed after OTC injection and oral administration. The OTC concentration in fed fish was the greatest after 10 d of feeding medicated feed, and was 0.46 and 0.14 µg·g·1in the 75 and 50 mg OTC·kg·1treatments, respectively. The OTC residue was below a detectable level in fish fed 50 mg·kg·1 10 d after treatment, and in fish fed 75 mg·kg·1 15 d after cessation of medicated feed. The B-value and half-life (tl/2) were 0.1642 ct·1 and 4.2 d, respectively, for 50 mg·kg·1fed fish; and 0.1759 d·1 and 4.0 d, respectively for 75 mg·kg·1 fed fish. Muscle OTC concentration was higher in fish receiving an intraperitoneal (IP) injection than in fish fed OTC. The OTC residue decreased from 7.56 µg·g·1 on the first day to 0.58 µg·g·1 on day 10 after injection for 163-mm fish, and from 5.83 to 0.27 µg·g·1 for 229-mm fish. Muscle OTC concentration was not detected in IP-injected fish 20 d after treatment in the samples of both size fish at 23.4°C. The B-value and t 112 were 0.2687 d·1 and 2.6 d for OTC-injected 163-mm fish, and 0.2554 ct·1 and 2.7 d for 229-mm fish. There was no significant difference in B-value and t112 between different sizes of fish injected with OTC. The OTC residue in muscle and liver of fish fed at rate of 1.5 and 3.0% body weight was significantly lower than fish receiving no feed.