Aquaculture production of rainbow trout has increased in Turkey in the recent years. There is great concern over the use of drugs for fish production. Bacterial diseases, if untreated, may lead to great economic losses in the form of fish mortality and reduced growth. Thus aquafarming uses a wide variety of drugs for prevention and treatment of disease in cultured fish. Among these drugs, oxytetracycline (OTC) is a broad-spectrum antibiotic used for the treatment of a variety diseases including vibriosis, furunculosis, enteric redmouth, bacterial haemorrhagic septicemia, fin rot, gill disease columnaris and edwardsiellosis (Black et al., 1991; Unb, 1996).