The numbers of methane-oxidizing bacteria (methanotrophs) in the sediments of Lake Washington were estimated using three culture-independent methods. Quantitative slot-blot hybridizations were performed with type I and type II methanotroph-specific probes. These data were compared to data from quantitative hybridizations using a pmoA-specific probe and a eubacterial probe. From the combined hybridi-zation data, the methanotroph population in Lake Washington was estimated to be 3.6 × 108–7.4 × 108 cells/g dry weight. Methanotroph community structure and number were also investigated using polar lipid fatty acid (PLFA) analysis. Analysis of biomarker PLFAs characteristic of both type I (16:1ω8) and type II (18:1ω8) methanotrophs was used to estimate the abundance of these bacteria in Lake Washington sediments. From the PLFA data, the methanotroph population in Lake Washington was estimated to be 7.1 × 108–9.4 × 109 cells/g dry weight. As a third method of quantitation, we calculated the methanotroph population using the total methane oxidation rate for whole cells in Lake Washington sediment to be 1.3 × 108–1.2 × 109 cells/g dry weight. The three independent estimates of the number of methanotrophs in Lake Washington sediment agree within a two- to fourfold range. These data suggest that the three techniques used in this study detect the functionally significant population of methanotrophs in Lake Washington. Furthermore, these techniques will be useful for obtaining estimates of methanotroph abundance in additional environments.