The USLE-MM is a rainfall erosion model which predicts soil loss using an erosivity index which is given by the QREI30 index to a power > 1.0. In the USLE modelling environment, soil erodibility is defined as the soil loss per unit quantity of the erosivity index. With the USLE-MM model where event soil loss = KUMM (QREI30)b1, using KUMM as the erodibility factor does not conform with the requirement that the erodibility factor has units that are consistent between soils and locations. The alternative is to use the model event soil loss = KUM a1 (QREI30)b1 where KUM is determined from dividing the total amount of soil loss from a large number of events by the total value of QREI30 associated with those events because this model isolates the soil erodibility effect from the effect of b1 on KUMM. The effect of b1 on a1 is demonstrated in this paper for bare fallow plots at 15 locations in the USA and plots at Sparacia, Sicily.