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Premise of the Study: Knowledge about the introduction history (source(s), number and size of introduction events) of an invasive species is a crucial prerequisite to understand invasion success and to facilitate effective and sustainable management approaches, especially for effective biological control. We investigated the introduction history of the Australian legume tree Acacia dealbata in South Africa. Results of this study will not only provide critical information for the management of this species in South Africa, but will also broaden our overall knowledge on the invasion ecology of this globally important invasive tree., Methods: We used nuclear microsatellite markers to compare the genetic diversity and structure between 42 native Australian and 18 invasive South African populations and to test different and competing introduction scenarios using Approximate Bayesian Computation analyses., Key Results: Australian populations were characterized by two distinct genetic clusters, while South African populations lacked any clear genetic structure and showed significantly lower levels of genetic diversity compared to native range populations. South African populations were also genetically divergent from native populations and the most likely introduction scenario indicated an unknown source population., Conclusions: Although we cannot definitely prove the cause of the observed genetic novelty/diversification in South African Acacia dealbata populations, it cannot be attributable to insufficient sampling of native populations. Our study highlights the complexity of unravelling the introduction histories of commercially important alien species., (© 2019 Botanical Society of America.)