Periphyton and phytoplankton productivities, chlorophyll a, carbon and nitrogen concentrations in experimental marshes flooded 1 m above normal level for 1 year and 2 years were compared to values for unflooded control marshes. Primary productivity was estimated from uptake of 14C~bicarbonate. Periphyton productivity was measured on artificial substrata that had been placed in the marshes. Phytoplankton productivity was estimated using samples of marsh water incubated in GO-ml glass bottles. All productivity measurements were made in the laboratory at 19°C and 15 jLE m-2 sec-1 of PAR, a low level of irradiance compared to field lev. els. Mean phytoplankton primary productivity, chlorophyll a, total suspended carbon, and total suspended nitrogen in control marshes were significantly higher than in flooded marshes. Mean periphytic primary productivity per unit area of artificial substrata was significantly higher in marshes flooded for 1 year than in control marshes; however, there was no significant difference between periphytic primary productivity of marshes flooded for 2 years and control marshes. There were no significant differences between control and flooded marshes in the amounts of periphytic chlorophyll a, particulate carbon, or nitrogen per unit area of artificial substrata. Flooding increased periphytic productivity and two of three measures of biomass, while phytoplankton productivity and biomass were reduced. Because the bulk of the nutrients were sequestered by the periphyton and metaphyton, we hypothesize that the increases in periphytic and metaphyton production associated with flooding were responsible for the decrease in phytoplankton production.