The turbulent fluxes of carbon dioxide and water vapor within rice plant canopies were measured over water surfaces which were covered and uncovered with duckweed (Lemna paucicostata). Observations were made by the eddy correlation technique with three-dimensional sonic anemometer thermometers and infrared carbon dioxide-humidity fluctuation meters. The amount of photosynthesis of an individual frond was measured with a portable chamber. The results are summarized as follows: 1) The turbulent flux of carbon dioxide at a height of 20 cm caused by the photosynthesis of duckweed was about -0.1-0.2mg/m2•s in daytime and about one-tenth of that in a rice stand. 2) The turbulent water vapor flux measured over the water surface covered by duckweed was slightly greater than that over the uncovered water surface. 3) The amount of photosynthesis of an individual frond was 0.14-0.17mg/m2•s at about 250-600, μmol/m2•s and 30°C conditions, and its value was nearly equal to that of the carbon dioxide flux above the duckweed community.