Cotton is the second most important cash crop contributing about 15% to the annual foreign earning in Tanzania and is purely from Gossypium hirsutum L. cultivars. Gossypium barbadense L., a textile source in other parts of the world occurs as a feral perennial of ornamental and medicinal value in home gardens. G. barbadense L. is a natural host of the red bollworm, a destructive pest to cotton. The Southern Highlands (SH) of Tanzania have been quarantined from cotton production to control spread of the red bollworm to other growing areas. Transgenic cotton expressing the delta-endotoxin genes from Bacillus thuringiensis (Bt) offers an alternative control to the pest and reduced dependence on insecticide. Gene flow between wild or valued feral relatives and transgenic crops is a biosafety concern should transgene escape result in resistance development and contamination of germplasm. Potential gene flow between feral G. barbadense (including accessions Gb1 and Gb2) from the SH and G. hirsutum cultivars was assessed using controlled hybridization. The crosses produced fertile F1 but intraspecific seeds from G. barbadense did not germinate. G. barbadense is more likely to receive than donate genes implying development of pest resistance if introgressed filial generations express the Bt product.