AbstractPrevious studies have revealed increased levels of germline mutations in minisatellite DNA in populations of herring gulls and mice sampled in Hamilton Harbour, Ontario Canada, an industrial area with high emissions of genotoxic contaminants like polycyclic aromatic hydrocarbons (PAH). In this study we investigated the occurrence of germline mutations of the highly mutable pentanucleotide microsatellite HrU10 in a population of free-living, migratory tree swallows breeding in the same area. We found a mutation rate of 5.3% (17/320 meiotic events) which was not statistically higher than that of a control population breeding in a rural and presumably unpolluted habitat in Eastern Ontario (3.4%; 14/412 meiotic events). In accordance with previous studies of microsatellite mutations, mutations were typical of slippage mutations with loss or gain of one (two) core repeat units, and the likelihood of mutation increased with the size of the microsatellite allele. We can not conclude that there is an effect of PAH on the HrU10 mutation rate, for that we would require a considerably larger data set to obtain statistical significance.