Concern that increasing atmospheric concentrations of CH 4 will contribute to global climatic change has stimulated research to assess the potential of soils to serve as a sink for atmospheric CH 4. Laboratory studies were performed to identify factors influencing CH 4 uptake by soils collected from a variety of ecosystems. Freshly collected samples from wetland and forest systems were all capable of depleting atmospheric concentrations of CH 4, whereas none of the samples collected from cultivated fields were capable of CH 4-uptake when incubated under near tropospheric concentrations of CH 4. However, exposure of cultivated soils to high concentrations of CH 4 (more than 1000 μl l −1) for several days induced uptake. When active soils were exposed to elevated concentrations, their initial rates of CH 4-uptake also increased. Incubation of two active soils for periods of more than 12 h under atmospheres containing CH 4 concentrations in excess of their K m values (51 and 192 μmol CH 4) caused an exponential increase in the rate of CH 4-uptake, whereas incubation under low CH 4 concentrations caused a rapid decline in activity. Little difference in CH 4-uptake was observed when H 2O content was adjusted between 25% and 75% of pore volume, but air-drying completely inhibited CH 4-uptake. Saturation of soil decreased the average rate of CH 4-uptake by 56%, presumably because of reduced gaseous diffusion. Optimal CH 4 consumption occurred in samples incubated at temperatures between 20 and 30°C and was greatly reduced at 40°C. Amending soils with clover residues reduced CH 4-uptake by an average of 43%. Also, addition of 7 μmol NH n4 + g −1 inhibited CH 4-uptake, whereas additions of 7 μmol NO 3 − g −1 had no effect. The inhibitory effects of NH 4 + appear irreversible and persist after nitrification of added NH 4 +. Because of the association of methanotrophic activity with CH 4 supply, recently drained or intermittently flooded soils are apt to display the greatest capacity for CH 4-uptake.