The radioactive fallout from the Chernobyl accident in 1986 contaminated parts of the boreal forest ecosystems in Sweden, and we report on the activity concentration of radiocaesium 137Cs in the meat of moose Alces alces caught in the county of Västerbotten in north-Sweden during 1986-1996. Countywide, the geographic distribution patterns of 137Cs activity in moose muscle were similar in 1986 and 1993. The underlying relationship between 137Cs concentration in moose muscle and ground deposition remained significant for all years, but the proportion of variation explained by this relationship was variable and low in most years. The transfer rate of 137Cs to moose underwent marked annual fluctuations that appear to be synchronous over large areas. The fluctuations in the uptake of 137Cs by moose most probably result from variations in food selection or shifts in habitats. The transfer rate of 137Cs to moose seems to be higher in coastal areas than in inland areas. The 137Cs activity in moose was considerably higher in 1993 than should be expected from a simple decay model based on original deposition data and the 137Cs levels in moose meat from 1986. The large temporal variations in transfer rate make future predictions of transfer to moose and man unreliable. We found that the annual hunting of moose is a major source of 137Cs transfer to man in this region.