Methane (CH4), the second most important greenhouse gas is mainly produced under reduced conditions such as submerged soils and is consumed by aerobic soils in the forest. It is generally considered that forest soils function as a CH4 sink. However, CH4 emissions may occur in humid temperate forests where wetlands are often formed in riparian zones. It is still not fully understood whether forest catchments act as net sinks or sources of CH4. We monitored CH4 fluxes at wetland and dry hillslope sites in a forested headwater catchment in central Japan. CH 4 fluxes were measured using the static chamber technique and several environmental factors were monitored. The study site was covered with mixed stands of secondary broad-leaved deciduous trees and planted coniferous trees. Several wetlands are located along the main stream of the catchment. The total catchment area and the wetland area are 59,900 m2 and 400 m, respectively. Most of the forest floor on the hill slope acted as a small sink throughout the year. In the wetland, CH4 was emitted almost throughout the year with clear seasonality. The average CH4 emission rates from wetland sites were 2-3 orders of magnitude greater than the consumption rate at the hill slope in summer. The CH4 emission rates and the dissolved CH4 concentration of soil porewater in wetlands were strongly dependent on soil temperatures and were highest during summer and lowest during winter. Although CH4 emissions increased markedly during most of the summer season, they were constrained by the depletion of the water table that facilitates oxygen transport from the atmosphere to the soil or by streamflow that increases the supply of dissolved oxygen. These results suggest that small wetlands in forest watersheds possibly function as a strong source of CH4 in the regions with humid and warm summers.