Phosphorus (P) losses from land-applied manure can contribute to nonpoint source pollution of surface waters. Dietary P levels influence P concentrations in animal manures and may affect P losses from land-applied manure. The objective of this study was to determine the effects of dairy diet P concentration on P losses in runoff from landapplied manure. Manures with P concentrations of 0.48 and 1.28 % P from two dietary P levels were applied to a silt loam soil 25 wet ton/a (36 and 96 lb P/a, respectively). The high P manure was also applied at 9.4 wet ton/a (36 lb P/a) to provide an equivalent P rate as the low P manure. Plots were subjected to simulated rainfall (2.83 in/hr) in June and again in October. Runoff was analyzed for dissolved reactive P (DRP), bioavailable P, total P, and sediment concentration. Natural runoff from the same plots was collected from November through May and analyzed for P as described above. At equivalent manure rates, DRP in June runoff from the high P manure was about 10 times higher (0.30 vs. 2.84 ppm) than the low P manure, and 4 times higher (0.30 vs. 1.18 ppm) when applied at equivalent P rates. Phosphorus concentrations in October runoff and November – May natural runoff were lower (0.02 – 1.69 ppm), but treatment effects were the same as for June runoff. These results show that excess P in dairy diets increases the potential for P loss in runoff from land-applied manure. Furthermore, diet P effects on potential losses in runoff from land-applied manure should be considered in P-indexing and nutrient management planning.