We examined rates of internal phosphorus (P) loading from sediments in Lake Pepin, an impoundment on the upper Mississippi River. Laboratory-determined rates of P release from the sediment, measured using intact sediment core systems, were very high, with mean values ranging from 3.8 to 15 mg m−2d−1under oxic and anoxic conditions, respectively. Independently-determined Fickean diffusional fluxes (mean of 4.9 mg m−2d−1), measuredin situunder oxic conditions using sediment peepers, were not statistically different from those rates determined in the laboratory under similar temperature, pH, and oxygen conditions. The dominant inorganic P fractions in the sediments were NaOH-extractable P and HCl-extractable P. Under both oxic and anoxic conditions, significant correlations existed between rates of P release and the NH4Cl- and NaOH-extractable P fractions, suggesting mat loosely-bound and iron- and aluminum-bound sediment P contributed to P release from the sediments in this lake. Correlations between total sediment P and extractable sediment P fractions indicated that loosely-bound and iron- and aluminum-bound P increased, while calcium-bound P decreased, with increasing total P content of the sediment. Our results suggest that internal P loading from die sediments under oxic, as well as anoxic, conditions can play a very important role in the P economy of this system.