The transfer of recombinant DNA molecules from the introduced organism to natural populations of bacteria may be an important factor in assessing the outcomes of planned release of genetically engineered organisms into the environment. As genetic transfer is further investigated, it appears that the potential for genetic transfer between bacteria of the same and other species and genera is much greater than had been suspected. At present, efforts are being made to limit the range of ecological niches available to genetically engineered microorganisms. Genetic transfer of recombinant molecules to natural populations of bacteria would nullify these efforts. (Copyright (c) 1988, Elsevier Publications Cambridge.)