Changes in juvenile rainbow trout (Oncorhynchus mykiss Walb.) gills at different times after transfer to a brackish water (4-7‰) culture in the Puck Bay were followed. Total gill tissue mounts as well as thin and ultra-thin sections were examined. Acute inflammation of the gill tissue, observed initially, receded after 2 weeks of adaptation. After 2 months in the culture, the gill structure was transformed, the transformations involving, i.e. increase in the activity and number of chloride cells and increase in the gill epithelium intercellular spaces.