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Made available in DSpace on 2022-04-29T00:13:27Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-08-22 The oxytetracyline (OTC) and florfenicol (FLO) are the most used antibiotics for bacterial disease control in fish. The aims are: estimating the lethal concentration 50% initial (LC(I)50;48h) of OTC (Terramicina ®) and FLO (Aquaflor ®) for pacu (Piaractus mesopotamicus); classifying OTC and FLO according to the acute toxicity and the risk of environmental intoxication; and assessing OTC and FLO dilution effect in water quality variables. Fish were acclimatized for 10 days in a bioassay room at 27 ± 2 °C. Then nine animals (three each concentration) were exposed to 7.0; 7.5; 8.0 or 8.5 mg L -1 of OTC or to 600.0; 700.0; 800.0; 900.0 and 1000.0 mg L -1 of FLO and a control treatment. The water quality variables recorded daily were: pH, temperature, electric conductivity, and dissolved oxygen. OTC and FLO were classified according to acute toxicity and environmental risk. The estimated OTC LC(I)50;48h for pacu is 7.6 mg L -1 and the estimated FLO is > 1000 mg L -1. OTC reduced the dissolved oxygen concentration, and it was considered moderately toxic, causing risk of environmental intoxication to pacu. FLO is not toxic; it does not cause risk of environmental intoxication to pacu and does not change water quality variables. UNESP Aquaculture Centre (Centro de Aquicultura da UNESP - CAUNESP), Via de acesso Paulo Donato Castellane s/n, CEP: 14.884-900, Jaboticabal, São Paulo University Federal of São Carlos (UFSCar), São Carlos Campus, São Paulo College of Agricultural and Veterinary Sciences at Unesp, Jaboticabal - São Paulo Ecotoxicology and Occupational Health Laboratory Laboratório de Ecotoxicologia e Saúde Ocupacional) College of Agricultural and Veterinary Sciences at Unesp, Jaboticabal - São Paulo UNESP Aquaculture Centre (Centro de Aquicultura da UNESP - CAUNESP), Via de acesso Paulo Donato Castellane s/n, CEP: 14.884-900, Jaboticabal, São Paulo College of Agricultural and Veterinary Sciences at Unesp, Jaboticabal - São Paulo Ecotoxicology and Occupational Health Laboratory Laboratório de Ecotoxicologia e Saúde Ocupacional) College of Agricultural and Veterinary Sciences at Unesp, Jaboticabal - São Paulo