The effects of four rates of straw mulching on runoff and soil loss were compared with those of no-tillage treatments under natural rainfall conditions using field runoff plots of 25 × 4 m established at 1, 5, 10 and 15% slopes on the International Institute of Tropical Agriculture (IITA) research site near Ibadan, Nigeria. The four rates of straw mulching were 0, 2, 4 and 6 t/ha. The mean annual runoff was 50, 10, 4 and 2% of the total annual rainfall for mulch rates of 0, 2, 4 and 6 t/ha, respectively. Runoff from unmulched treatments was not related to slope. Runoff loss from no-till treatments was only 2% of the rain received. The mean soil losses for the rainstorms greater than 25 mm were 143, 16, 2 and 0.4 kg/ha per mm of rain received for mulched rates of 0, 2, 4 and 6 t/ha, respectively. The soil loss declined exponentially with increasing mulch rate with exponents ranging from approximately −0.3 to −0.7. The soil losses from the no-till plots were equal to those from plots that received mulch at the rate of 6 t/ha. Soil erodibility was significantly influenced by time after clearing, with maximum K reached two to three years after forest removal. The nutrient loss in runoff and eroded soil was significant only for unmulched treatments. The maximum annual loss of NO3-N in runoff was about 15 kg/ha. The maximum annual loss of total N in eroded soil from unmulched plots was about 180 kg/ha, that of P, 5 kg/ha, and that of K, about 14 kg/ha.