The concentration of CH4 in 131 groundwater samples used for agriculture in Aichi Prefecture, central Japan, averaged 1.58 mgC l-1 for those water samples above detection (>0.006 mgC l-1), with the highest value of 18.4 mgC l-1. Methane was detected in more than half of the groundwater samples. The amount of CH4 released to the atmosphere because of agricultural groundwater use was estimated to be 2.00¿107 gC yr-1 for a cultivated area of 8.61¿104 ha, or about 1.4% of the CH4 production in paddy fields derived from soil organic matter in the same geographic area. Distribution of measurements of redox potential (Eh), chemical oxygen demand (COD), Fe, Mn, NH4-N, and NO3-N was clearly different between the CH4-detected and undetected samples; Eh values and NO3-N concentrations were lower while the other four factors were higher in the CH4-detected samples. ¿ American Geophysical Union 1994