To determine the effect of oxolinic acid (OA) dosage treatment on the selection of antibiotic-resistant bacteria in farmed trout, three different dosages of OA were administered over seven consecutive days, via medicated feed, to healthy rainbow trout reared in experimental tanks supplied with river water. OA dosage licensed in France (12 mg/kg/day), over-dosage (24 mg/kg/day) and under-dosage (6 mg/kg/day) were tested. The importance of trout gut and rearing waters as potential sites of selection for antibiotic-resistant bacteria in fish farming were investigated (i) by quantification of heterotrophic microbiota on GSP selective medium and of motile Aeromonas on TSA medium, and (ii) by characterisation of Aeromonas OA susceptibility levels before, during and until two weeks after treatment. In the intestines of treated and control fish, bacterial counts were severely reduced during the entire experiment, and Aeromonas populations remained susceptible to OA. In contrast, bacterial counts in output water of both medicated and control tanks were significantly higher than in input water. OA minimum inhibitory concentration distributions of Aeromonas estimated with a replica plating method (GSP plates supplemented with OA concentrations from 0.03 mg/l to 2 mg/l) indicated that the treatment, whatever the dosage, had induced the emergence of OA-resistant Aeromonas in output water. OA MIC of resistant Aeromonas isolated on GSP agar (estimated MIC ≥ 4 mg/l) determined using an agar dilution method on Mueller–Hinton agar confirmed the presence in output water of highly OA-resistant Aeromonas (MIC ≥ 128 mg/l). Between one and two weeks after the end of treatment, those highly OA-resistant Aeromonas were all the more prevalent among total Aeromonas populations that treatment dosage was higher. Results suggested that selection and emergence of OA-resistant Aeromonas had probably occurred in faecal matters after excretion, rather than in the fish intestines.