The release of Bt cotton varieties genetically modified for increased tolerance to major cotton insect pests provided impetus for the reestablishment of a cotton industry in northern Australia. However, this stimulated concern that the addition of the gene might facilitate an increase in the potential for weediness of genetically modified cultivars in noncropping habitats. Bt and conventional cottonseeds were planted in 12 sites in northern Australia to test the hypothesis that there would be no increase in the ability of Bt cotton vs. conventional cotton to establish weedy, or invasive populations, defined as population growth over time greater than one, irrespective of location, habitat, seed type, or population density. Invasiveness was a factor of germination, survival, and recruitment. We examined whether the addition of the Bt gene would increase fitness of these parameters, and associated invasiveness. An irrigation drain was considered a high-risk habitat for cotton establishment, so an additional site was sown at that habitat to provide supplementary data to the original 12 sites. Location and habitat were the dominant factors influencing germination, survival, fecundity, and invasiveness. Bt and non-Bt cottonseeds did not differ in their ability to germinate, establish, and survive. After 2 yr, cotton plant survival was very low, and only 3 of 13 sites established fecund cotton populations. Measurements continued for an additional 2 yr at these sites. There was no increase in values for invasiveness for the Bt genotype treatments at any location or habitat after 2 yr or at two selected habitats after > 4 yr, demonstrating that the addition of the Bt gene will not confer increased fitness for weediness. Mean invasiveness values for each habitat, irrespective of genotype, were less than one, indicating that neither conventional nor Bt cotton would establish invasive cotton populations in northern Australian habitats.