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The seasonal dynamics of small mammal populations was studied in an isolated 2.6-ha woodlot surrounded by agricultural "desert" in central Germany in 1984-88. The CMR method with monthly live trapping was used in 1984-86. In the autumn of 1986 all captured animals were removed and the recolonization of the woodlot was monitored until the spring of 1988. The bank vole (Clethrionomys glareolus) and wood mouse (Apodemus sylvaticus) were dominant species in the woodlot. Wood mice exhibited a seasonal habitat change between woodlot and field, together with the less abundant yellow-necked mouse (A. flavicollis). The striped field mouse (A. agrarius) normally occurred in the forest in low numbers. During high densities of the common vole (Microtus arvalis) on the surrounding fields it also penetrated the inner parts of the woodlot. Contrary to expectation Clethrionomys was the most effective colonizer of the woodlot after the removal of rodents. The sex ratio of Clethrionomys was 1:1 at all seasons, that of Apodemus-species male-biased during the autumn immigration. The total densities of the rodent community were much higher during the autumn and winter than during the breeding season. The number of shrews was very low during the entire study and these seem to have a poor ability to colonize isolated areas. The study supports the idea of the importance of isolated woodlots in the agricultural mosaic, especially as overwintering refugies for rodents.