1. The population fluctuations in time in northern Sweden are examined for the following species: voles, mountain hare, willow grouse, black grouse, capercaillie, hazel hen, red fox, long-eared owl, Tengmalm's owl, and tularemia. Necessary population data have been obtained from the period 1963-1975/76 as revealed by catches, literature survey, hunting statistics, bird ringing, and obligatory reporting of tularemia in man. 2. The populations of the species under consideration are found to fluctuate synchronously in time and show a 3- or 4-year cycle for the period 1963-1975. Population peaks have occurred in connection with the peak densities of voles in the winters 1963-1964, 1966-1967, 1969-1970 and 1973-1974. 3. Voles caused extensive forest damage (mainly bark-eating) in at least the latter three peak winters. From consideration of the available literature it is apparent that bark is a marginal food. Thus, increased bark-eating during peak densities of voles in winter should be interpreted as a shortage of preferred food. 4. The species studied appear to form a unit (subsystem) within the boreal forest ecosystem. This idea is supported by the connecting predatorprey relationships and the demonstrated synchronous population fluctuations. The subsystem contains herbivores, their food vegetation, and predators. Tularemia is regarded as only one among other predators on voles and mountain hares. 5. It is postulated that voles play a central role in causing the overall synchronism in the population fluctuations of the subsystem. 6. The synchronous population fluctuations described can be explained by the following model for their regulation: a) An initial decline in vole numbers is brought about by food shortage at winter peak densities. b) Predator populations (built up with the help of the rich supply of voles) cooperate with food shortage and at some critical point predators alone are able to fulfil the decrease in vole numbers. c) Because of the decrease in vole numbers the predators are forced into a decline themselves and must turn to alternative prey species. Mountain hare and gamebird populations represent a low biomass compared with vole populations and predation thus causes the decline in numbers of these small game. d) Low numbers of predators and excessive food supply then allow voles, mountain hares, and gamebirds to increase again. e) The building up of vole populations sets the stage for another increase in the number of predators and a new cycle is started.