The tropic structure and the function of a small pone ecosystem under the tree stand were studied in terms of energy flow. About 28% of total solar radiation was intercepted by the tree canopy over the pond. Primary producers converted 1.1%(3,382 kcal$cdot$$m^{-2}$$cdot$$y^{-1}$) of solar radiation (320,000 kcal$cdot$$m^{-2}$$cdot$</TEX>$y^{-1}$) into gross primary production. The amount of energy availble to the pond snail was 1,683 kcal.m-2.y-1 of the net production by primary producers and 1,033 kcal$cdot$$m^{-2}$$cdot$</TEX>$y^{-1}$ of the litter fallen into the pond. The amount of gross secondary production by the pond snail was 245 kcal$cdot$$m^{-2}$$cdot$</TEX>$y^{-1}$. Judging from these, supply of both net primary production and the litter was indispensable for the maintenance of the pond ecosystem. The total amont of energy as gross primary production plus litter was 4,415 kcal$cdot$$m^{-2}$$cdot$</TEX>$y^{-1}$(100%). Since the total respiration loss was calculated to be 1,917 kcal$cdot$$m^{-2}$$cdot$</TEX>$y^{-1}$(43.4%), the rate of energy accumulation in the pond estimated to 56.6%.