The major sources of phosphorus from agricultural activities are (1) surface runoff from cropland (2) runoff from livestock operations including runoff of winter spread manure (3) streambank erosion and (4) runoff from unimproved land. This report presents the combined results of several studies in the agricultural watersheds in terms of the total agricultural contribution and the contribution from each of the four major sources. The information obtained from the agricultural watersheds has been extrapolated to provide estimates of the contribution from each source in the Grand and Saugeen River basins. Finally, the information has been extrapolated to provide estimates of the total contribution from agricultural sources in the remaining regions of the Southern Ontario portion of the Great Lakes Basin.