In order to known the extend of contribution to the degradation of organic materials and nutrient recycling by sulfate-reducing bacteria (SRB) and methane-producing bacteria (MPB) in sediment, the distribution and activity of these two groups of microorganisms were studied montly in 1994 at two sites, one littoral (Sanggulri) and the other profunndal (DAM), in Lake Soyang. In the seasonal distribution of two microorganisms, SRB were 1.07 $ imes$ 10$^{3}$-2.42 $ imes$ 10$^{5}$ cells/g-dry weight at Sanggulri, 2.40 $ imes$ 10$^{5}$ -1.29 $ imes$ 10$^{6}$ at Dam and MPB were 0.52 $ imes$ 10$^{3}$ cells/g-dry weight at Sangguri and 1.44 $ imes$ 10$^{3}$-6.89 $ imes$ 10$^{3}$ at Dam. In these results, the density of SRB in Lake Soyang is much higher than other lakes. These high values might be due to higher sulfate concentration, 0.69-4.05 mM, than normal freshwater, 0.01-1.2 mM. And a good correlation of SRB and chlorophyll a concentration implied that the important environmental factor on distribution of SRB might be the concentration of available organic matter. In a comparison of sulfate-reducing rate and methane producing rate in 1995, the activity of SRB for the degradation of organic matter was higher than MPB by factor of 359. Conclusively SRB superior to MPB in the distribution and activity are more important annearobic bacteria in Lake Soyang sediments.