To understand why atmospheric methane is increasing worldwide, accurate estimates are needed of the global input from rice fields. We report greenhouse and laboratory studies over three growing seasons to isolate and control factors that might affect methane emission from rice paddies, including soil texture, added exogenous organic matter (OM), nitrogen and sulfate ion, and water management. Without added OM, methane production was relatively low, increasing during the growing season, and continuing after harvest, provided the soil remained water-logged. If ground rice straw was added to the soil prior to planting methane production began shortly after flooding, with an initial burst of the gas after 3 to 5 weeks, and then a gradual increase to a second peak later in the season (and after harvest), with rates considerably higher than in treatments without added OM. The initial methane burst was largest when the rice straw was added near the surface. We conclude that exogenous OM is the major contributor to methane production, with at least two distinct reaction patterns involved. Emitted methane accounted for 5% or more of the added organic carbon. Methane emissions were from 3 to 12 times higher with added OM than from identical soils without such additions with largest emissions when OM is added deeper in the soil. Methane release as bubbles was common (the main source in the absence of vegetation), but the plant normally was the main conduit for gas transfer to the atmosphere. Addition of sulfate ion (gypsum) to the soil resulted in a slight competitive suppression of methane production. ¿ American Geophysical Union 1993