Abstract. The efficacy of florfenicol in the control of natural outbreaks of furunculosis in Atlantic sallmon, Salmo salar L. was evaluated on five Norwegian farms during the summer of 1992. A total of 115 245 first- or second-generation fish received florfenicol with feed at a dose rate of 10mgkg 1body weight daily for 10 consecutive days. Comparisons were made with similar pens of fish medicated with the preferred treatment for eaeh location. In the 10 days following introduction of therapy, all treatments were associated with a rapid decline in mortality caused by infection with Aeromonas salmonicida. Florfenicol-medicated salmon had a lower specific mortality than those given oxolinie acid, flumequine or a trimethoprim and sulphadiazine eombination. No adverse reactions to treatment with florfenicol were observed.