The plasma concentrations of florfenicol (FF) were measured during a 10-day oral administration of this agent to fish in a cage in a commercial salmon farm. The therapy was initiated as part of a management response to an outbreak of winter ulcer disease associated with a Moritella viscosa infection. Twenty healthy, 20 moribund and 10 dead fish were sampled 24 h after the administration of medicated feed on the eighth day of the therapy. Quantifiable concentrations of FF were detected in all 20 healthy fish with a mean plasma concentration of 3.0 +/- 1.8 mg/ml. These data are consistent with previously published data and with parallel laboratory trials performed in this work. The mean weight of these healthy fish was 324 89 g but M. viscosa was isolated from only one of them. With respect to the moribund fish, FF could not be detected in 18 of the 20 fish analysed and the mean weight of these fish was 196 48 g. M. viscosa was isolated from 19 of the 20 moribund fish.The mean MIC value for FF against 14 of the M viscosa isolated was 0.24 mg/l and it is argued that, in the context of this therapy, these bacteria should probably be classified as sensitive to FF. The analysis of the patterns of mortality before, during and after the period of therapy did not provide any unambiguous evidence that the therapy was, however, effective in reducing death in the cage population.The data generated in this work and in a previous study of winter ulcer disease is used to develop a tentative model of the epizootiology of this disease in post-smolts. In both studies there was a strong correlation between the weights of fish and their health. This suggests that, within a cage population, infection by M. viscosa, ulceration and death are largely confined to a sub-population that has adapted poorly to the environment of the marine cage. Further, as this poorly adapted sub-population is not feeding, it is unlikely that orally administered antimicrobial therapy would represent a cost-effective way of limiting losses. (C) 2005 Published by Elsevier B.V.