Mass balances for total phosphorus (TP) were measured for a minimum of 9 years in 8 lakes located near the southern boundary of the Canadian Precambrian Shield in Ontario. The lakes ranged from having no shoreline development to having intensive development. Potential inputs of TP from anthropogenic activities ranged from 0% (in the undeveloped lakes) to about 60% of the TP load. In all the lakes, internal sources of TP were small (generally less than 10% of the total TP load). Atmospheric deposition was a major source of TP (approximately 21 mg/m2/yr) in those lakes with small catchments relative to lake area. Export of TP from the watershed was important in those lakes with large catchment: lake area ratios.