Major advances have been made recently in the application of molecular biological techniques to environmental settings. The methane oxidizing bacteria, methanotrophs, are ubiquitous in nature and grow on methane as their sole source of carbon and energy. They have important roles in both the global carbon cycle and the degradation of hazardous compounds. In this study, the natural populations of methanotrophs in the sediments of Lake Washington and their role in the degradation of trichloroethylene were examined. A number of molecular techniques were developed to aid in the analyses. Using these techniques, the methanotrophic populations present in the area of peak methane oxidation in Lake Washington sediments were characterized. The data obtained suggest that the methanotrophs detectable using a combination of genetic tools demonstrate a diversity as broad as the known methanotrophs from all mesophilic environments. These results are in contrast to other environments, such as peat and marine environments, that appear to be dominated by a limited diversity of methanotrophs. Furthermore, the data suggest that current bioremediation strategies may not enhance trichloroethylene degradation by methanotrophs.