The goal of the Terrestrial Microbial Ecology/Biotechnology Program is to develop methods to evaluate potential impacts of genetically engineered microbes (GEMs), and microbial pest control agents (MPCAs), on terrestrial ecosystems. In the last few years, biotechnology companies have constructed genetically engineered viruses, bacteria, and fungi that will function following their purposeful release into the environment. The technology has brought a renewed interest in microbial ecology and a need to define and better understand the nature of exposures and possible risks of releasing these novel microbes. The authors efforts have dealt largely with the development of complex microcosms to study the kinetics of conjugal gene transfer, survival, and dispersal of various bacterial genera containing recombinant DNA. They have found that insects can serve as reservoirs of recombinant organisms and as vectors causing contamination of plants and soil. Evaluation of the data on aerosol dispersal of recombinant bacteria spray released in California revealed that simple monitoring devices can be used to document the extent of bacterial dispersal. Ultimately, data from laboratory, microcosm, and field evaluations will be integrated into mathematical models that will help predict the ramifications of releasing recombinant microbes into the environment.