Technical Abstract: During the century following its initial introduction in 1886, the Australian tree Melaleuca quinquenervia(Myrtaceae) dispersed from a few introduction points to occupy over 200,000 ha, primarily in historic Everglades wetlands of southern Florida. Cellulose acetate gel electrophoresis (CAGE) was used to investigate the allozyme diversity and population genetic structure of 208 individuals in a dozen populations resulting from this invasion. The analyses showed that these populations have a high (82%) rate of polymorphic loci and an average of 2 alleles/locus. There was substantial heterozygosity (mean He = 0.356), which concords well with recent studies reporting a greater number of introduction events and sources than generally recognized. The introduction history and distributional patterns within Florida have led to geographic structuring (GST = 0.419) in which the Gulf Coast metapopulation has a greater effective number of alleles and greater heterozygosity than the Atlantic Coast metapopulation. The gene diversity in M. quinquenervia was comparable to other tropical woody species. Its strong population divergence was reminiscent of pioneer species and consistent with its status as a plant invader in Florida.