Provided here is an up-to-date survey of gene transfers inthe main natural habitats, with a special reference togenetically engineered microorganisms.In the first of fivesections technical approaches of gene transfer in thenatural environment are developed. These emphasize the useof modernmethodologies for the detection of recombinantbacteria in natural waters and soil, using DNA or rRNAprobes and PCR technology. The three following sections dealwith various aspects of gene transfer in aquaticenvironments, terrestrial habitats and human and animal gut.These include transfer of plasmidic or chromosomal markersthrough either conjugation, mobilization, transformation ortransduction processes. Also covered are factors influencingsurvival of cells harbouring the transferred genes in theseenvironments. The last section is devoted to an examinationof scientific and ethical problems related to the release ofgenetically engineered microorganisms.