Phosphorus budget calculations for Shagawa Lake show that significant increases in lake P during July and August 1971-1975 originated from within the lake basin. Mass increases of 2,000-2,900 kg (35-50 ug/liter) were measusred, corresponding to internal supply rates of 240-680 kg/wk over a 3-9 week interval. After anaerobic conditions developed in late June, release from the profundal zone sediments contributed significant amounts of P to lake water. Sediment chemical properties were determined to estimate the size of the P reservoir in the profundal zone. Although external sources of phosphorus have been reduced by about 80% the feedback of P from the sediments is supporting the productivity of Shagawa Lake and may continue to do so for many years.