Atlantic salmon were held in experimental tanks and oxytetracycline-HCl was administered at 112.5 mg/kg body weight per day for 12 days via medicated feed. The frequency of resistant strains, defined as those capable of colony formation on 2216V medium containing 25 mu g/ml oxytetracycline, was monitored in samples taken from the intestinal contents of the fish (n = 5). No evidence for a selection of resistant strains in these intestinal microflora was detected either during the period of administration or in the subsequent 16 days during which the fish were fed unmedicated feed. The range of the mean frequencies of resistance of the intestinal flora in the period after medication (0.1%-9.9%) were always lower than the frequency of resistance in the microflora of the feed (16.0%) which was fed to them. The analysis of the water samples (n = 5) obtained on each sampling day showed that frequency of resistance increased significantly during the experimental period (28 days). It is argued that the high frequencies of resistance that were detected in some of the water samples taken towards the end of the experimental period may have been the result of increasing accumulation of uneaten feed in the tanks, rather than a consequence of the presence of oxytetracycline. (C) 1997 Elsevier Science B.V.