The goal of the project was to develop new field trial methods needed to generate data for initiation of the drug registration process specific to aquatic cultured fish. Currently, there is a shortage of safe and effective antimicrobial drugs to treat diseases in cultured fish and no validated methods for clinical field trials. Difloxacin, a drug used to treat furunculosis in Atlantic salmon, was used to treat fish in both small scale and production size net pens. The modified net pen showed no significant differences in mortalities between medicated and control fish. However, the mortality rates for rated fish could not be fully assessed because severe outbreaks of furunculosis did not occur during the study period. Operational requirements were developed for use of the small net pen and recommendations were made for further refinements.