Despite considerable research, large uncertainties exist in the various area-scales budgets of CH4 flux between soils and the atmosphere. We postulated that small wetlands greatly contribute to the CH4 budget in semiarid regions, where wetland contributions are often assumed to be insignificant. To test this hypothesis, small riparian mires of the Xilin River basin in Inner Mongolia were selected as a case study for weekly measurements of in situ CH4 flux. Average CH4 fluxes were 234.3 and -1.9 mg CH4 m-2 d-1 for the riparian mires and the upland grassland, respectively, during July--October 2003. Although the riparian mires cover only 0.4% of the Xilin River basin, their total CH4 emission was about half of the amount of CH4 consumed by the upland grassland with 89.7% land cover ratio. Our results indicate that estimated CH4 budgets for semiarid regions should include an assessment of small wetlands.