The effect of initial sediment concentration on settling velocity distributions measured by the modified bottom withdrawal tube (MBWT) method is presented for a well-aggregated clay soil (a vertisol). Sediment concentrations in the experiments range from 10 to 100 kg m (super -3) of suspension. Particle Reynolds numbers in the polydisperse suspensions range from less than 0.1 up to 100. Settling velocities of all size classes in the sediment are reduced by an increase in sediment concentration. The empirical equations of Richardson and Zaki appear useful for predicting ratios of settling velocity at different polydisperse concentrations, but absolute velocities predicted are lower than measured. The discrepancy is attributed to the empirical equations being derived for ratios of particle Reynolds numbers much lower than are present in this natural sediment. Hydrodynamic wake capture occurs during this sedimentation, a process not considered in the empirical equations. Settling velocities of all size classes in the sediment are also reduced by a decrease in MBWT diameter, and an incorrect relation between settling velocity and sediment concentration is measured using the "small" tube originally recommended for this technique. A minimum tube diameter of 45 mm is recommended to avoid such errors.