Technical Abstract: The size distribution of sediment affects erosion rates, settling velocity, and the transport of chemical constituents. Little information is currently available concerning the size distribution of materials transported in runoff from beef cattle feedlots. The objectives of this study were to: a) measure the size distribution of materials transported in runoff from feedlot surfaces containing varying amounts of unconsolidated surface material (USM), and b) determine the effects of varying runoff rate on sediment size distribution. Simulated rainfall was applied to 0.75-m wide by 2-m long plots located within feedlot pens. Sieve and pipette analyses were used to measure the diameter of the eroded materials. For each of the treatments with varying amounts of USM, D50 values, the size for which 50% of the sediment is smaller, were less than 63 µm. Significant differences in D50 values were found between the feedlot surfaces containing 0 or 6.7 kg ha-1 of USM and those with 13.5 or 26.9 kg ha-1 of USM. The proportion of larger size sediment fractions transported in runoff generally increased as runoff rate became greater. No significant differences in D50 values were found for runoff rates varying from 0.5 to 9.7 kg min-1. The D50 value of 310 µm obtained at a flow rate of 15.3 kg min-1 was significantly greater than measurements for the other runoff rates. The size distribution of sediment from feedlots is influenced by both the amount of unconsolidated surface material and runoff rate.