Black rats were introduced to Australia just over 200 years ago and now occur across the entire continent,most typically associated with human settlements but also as feral invaders of natural habitats.We sourced black rats from all major regions of Australia but undertook our most intensive sampling in South Australia.We examined mtDNA sequence diversity for all samples and Y-chromosome sequences and microsatellite DNA for a subset.Our results demonstrate that Australian black rats contain genetic components derived from both typical ship rats and Asian house rats,and suggest a complex history of multiple introductions.Mismatched associations between ship rat and Asian house rat mtDNA and Y-chromosomes probably result from introgression following introductions.Microsatellite DNA analyses indicate that all Australian black rats possess an autosomal genome derived largely from a ship rat ancestry but also indicate local barriers to gene flow between populations of contrasting mtDNA identity.The broader implications of these findings for black rat invasion histories and futures will be discussed.