The seasonal flux of methane to the atmosphere from salt marsh soils was examined in three different vegetation zones within a single marsh near Yorktown, Virginia. A total of 100 measurements were made over a 2-year period, with maximum rates occurring during summer and fall. Spatial and temporal variability influxes was high; rates ranged from -2.4 to 21.3¿10-3 g CH4/m2/d. Estimates of annual methane losses to the atmosphere were 0.43 g CH4/m2 for a salt meadow zone, 1.3 g CH4/m for short Spartina alterniflora, and 1.2 g CH4/m2 for tall creek-bank S. alterniflora. A total of 63 flux measurements, made in a variety of other coastal salt marshes along the east coast of the United States, suggest that the Virginia site may be fairly typical for this region. In addition to diffusional losses across the air-soil interface, methane can be lost from the marsh system to the atmosphere through the lateral movement of pore waters supersaturated with methane into tidal creeks, with subsequent degassing across the water-air interface. Estimates of the magnitude of methane input to the atmosphere by this mechanism indicate it may be as important as diffusional losses across the air-soil interface. These data suggest that salt marshes of this type make only a minor contribution of global atmospheric methane.