Genetic similarity within pairs of individuals was examined using both 10 polymorphic microsatellite loci and multi-locus DNA fingerprinting profiles in a semi-isolated population of great reed warblers at Lake Kvismaren, south Central Sweden, in 1987-1993. The population was founded by a few individuals in 1978, followed by a gradual increase in numbers until 1988, since when the population has remained relatively stable with about 60 breeding birds. We have previously found that high genetic similarity between pairmates in the population during the early part of the study period reduced egg hatching success, and hence reproductive success. The measures of pairwise genetic similarity, microsatellite allele sharing and DNA fingerprinting band sharing, were highly correlated with pedigree-based relatedness. Both microsatellite and DNA fingerprinting similarities between pair-mates declined significantly over the study period, and the pattern was most pronounced in the DNA fingerprinting data. Analyses restricted to the microsatellite data showed that the average annual microsatellite similarity between pairwise combinations of individuals, as well as individual homozygosity in males, declined significantly over the study period, and that several immigrants carrying novel alleles entered the population during the study. Hence, the temporal decline in genetic similarity of mates in the population is probably a consequence of increased immigration, facilitated by the recent expansion of the species in the region. These results suggest that the population has now recovered genetically, or is in the process of recovering, from a recent founder event.