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Singapore is a recognized global hotspot for invasive species and many introduced plant species have become major weeds there. Some of the common invasive taxa, such as Asystasia gangetica ssp. micrantha, Mimosa pigra, Neptunia plena, Panicum maximum, and Urochloa mutica, are spread over large areas and dominate the indigenous flora in some habitats. In a study aimed at understanding the relationship between polyploidy and invasiveness, we show that all the investigated invasive taxa are polyploids. A. gangetica ssp. micrantha, N. plena, and P. maximum vary in chromosome number and ploidy level across the world, but we recorded only one chromosome count for each of these species in Singapore. Similarly, the cytology of M. pigra and U. mutica also revealed that these species are polyploid, each with only one chromosome number across all populations. The results indicate that one polyploid line in each of these species has been selected favourably and has become invasive. We also show that all the species exhibit normal male meiosis and possess high percentages of pollen fertility. Based on the present study and an analysis of previously reported ploidy levels, we suggest that these taxa are probably of allopolyploid origin. We conclude that polyploidy and an effective reproductive system are a perfect mix for successful invasion by these species in Singapore. © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society, 2006, 151, 395–403. [ABSTRACT FROM AUTHOR]