The population of adult cisco in the Bothnian Bay consists of a number of spawning populations. Each one of these returns to the same inshore area for spawning every autumn. In the summer these populations are mixed and occur mainly offshore. These are the main conclusions of an experiment in which cisco were marked with Carlin-type tags. The cisco were caught mainly in fyke nets both in the summer and in the autumn and released after tagging in the area where they had been caught. Fish tagged in the autumn and recaptured in the autumn one or more years later showed a strong affinity to the point of release, while fish from the same group recaptured in the summer had migrated far from the release site. The distribution of cisco that were tagged in the summer and caught in the autumn showed no correlation with the release site.