Direct measurement of an interrill erodibility factor (Ki) is costly and time intensive. As Ki and the final infiltration rate (FIR) under seal formation are both affected by aggregate breakdown at the soil surface, it was hypothesized that the Ki and FIR values are correlative. FIR and soil-loss values of 53 soils, measured in several different laboratory rainfall simulators, were investigated. The slope factor (Sf) of smectitic soils was higher than that of nonsmectitic soils at slope angle (&thgr;)>9%. The equation Sf=exp(-0.68+8.28 sin &thgr;) defines significantly (r2=0.94) the Sf for smectitic soil. For the various soils the FIR and Ki values were correlative and fitted significantly the Ki=a-b ln(FIR) model; a and b are empirical coefficients. This model was found applicable for a wide range of rain intensities (34--68 mm h-1). However, an increase of the raindrop kinetic energy from <11.6 to 22.3 J mm-1 m-2 increased the absolute values of the coefficients, a and b. The FIR-Ki model differed for smectitic and nonsmectitic soils; at a given FIR, the smectitic soils had a higher Ki value than the nonsmectitic soils.¿ 1997 American Geophysical Union