AbstractAn epidemiological longitudinal study comprised information obtained from 124 randomly selected Norwegian sea‐sites housing post‐molts of Atlantic salmon,Salmo salarL. Information on crude cumulative mortality, fish characteristics and farm factors was gathered by the veterinary practitioners servicing the farms. The observation period lasted from the time of sea transfer of the smolts in the spring until 1 October 1991. On average, 116480 smolts were transferred to each sea‐site. The mean crude cumulative mortality of post‐smolts during the period concerned was 16.8%. The cumulative farm‐level incidence was 54.0% for furunculosis, 39.5% for infectious pancreatic necrosis (IPN) and 10.5% for vibriosis. Occurrence of any of these specific infectious diseases in the post‐smolt was significantly associated with mortality. The risk of furunculosis was significantly related to the location of the sea site. Mixing of smolt from many freshwater hatcheries in the sea farm increased the risk of IPN significantly. In addition, the risk of IPN was related to age of site and geographic location. The mean cumulative mortality in 1‐year‐old smolt groups was 17.5%, significantly greater than in two‐year‐old fish (7.9%). In addition, the method used to transport fish from the freshwater hatchery to the sea site was significantly associated with mortality. Neither the weight of smolts at sea transfer nor the date of transfer were significantly r