In order to develop a soil organic carbon.(SOC) indicator for Eastern Canada the Century model was run for all Soil Landscape of Canada (SLC) polygons in the Mixedwood Plains Ecozone of Ontario. Within each polygon, SOC dynamics were estimated for three or four crop rotations and two tillage practices. It was determined that the Mixedwood Plains Ecozone was losing SOC at the rate of -39.6, -34.9, -32.1, and 24.3 kg C ha-1 y-1 for the years 1981, 1986, 1990, and 1991, respectively. Such rates of loss are not measurable on a year to year basis and are usually considered to be at near equilibrium. The introduction of no-till agriculture is the principal factor that reduced the rate of loss of SOC. There was greater loss of SOC near the Southern tip of Ontario where agriculture is most intensive (less hay and pasture). The greatest loss of SOC was found in corn and soybean rotations and there was a gain in SOC under hay and pasture rotations. It was found that soil texture influences SOC dynamics with more SOC being lost in coarser textured soils. The Century model did a reasonable job in estimating SOC dynamics in Ontario. The model was developed and parameterized in the western US grasslands so it was expected to be more sensitive to western Canada SOC dynamics. Considering that the model has been tested successfully worldwide under various climatic, soil, and management practices there is likely no better tool for developing a soil carbon indicator at the present time.