Benthic green macroalgal mats, composed primarily of Enteromorpha spp., were studied on a mudflat in the Coos Bay estuary (Oregon. USA) during 1981 and 1982. Monthly field collections indicated highest biomass during August of both years. From April to September, maximum algal abundance progressed from low-to mid-intertidal elevations until storms physically removed algae from the mudflat. Annual production was estimated at 1100 g C m-2 by integration of (1) field measurements of standing crop, light, salinity, and temperature; (2) laboratory measurements of carbon fixation rates under varying conditions of desiccation, light, salinity, and algal density; (3) computergenerated estimates of tidal emersion and submergence for the Coos Bay estuary. Submerged photosynthesis accounted for an average of 95 % of total production despite light attenuation by the water column at the lowest intertidal elevations sampled. Emersed production was restricted by desiccation, light attenuation within the compressed algal mat, inorganic carbon limitation, and decreasing time of daylight emergence through the growing season. Enteromorpha contributes to the estuarine production chiefly through release of dissolved organic carbon, burial of algae in sedlments, herbivore-enhanced fragmentation, and whole-plant loss.