Provided here is an up-to-date survey of gene transfers in the main natural habitats, with a special reference to genetically engineered microorganisms.In the first of five sections technical approaches of gene transfer in the natural environment are developed. These emphasize the use of modernmethodologies for the detection of recombinant bacteria in natural waters and soil, using DNA or rRNA probes and PCR technology. The three following sections deal with various aspects of gene transfer in aquatic environments, terrestrial habitats and human and animal gut. These include transfer of plasmidic or chromosomal markers through either conjugation, mobilization, transformation or transduction processes. Also covered are factors influencing survival of cells harbouring the transferred genes in these environments. The last section is devoted to an examination of scientific and ethical problems related to the release of genetically engineered microorganisms.