Six structural indices at three micro and three macro levels were used to measure the stability of 15 natural aggregates to water. Of the micro tests (the percent water-stable aggregates WSA < 0.25 mm) index reflected differences in organic carbon (O.C.) levels more than the aggregated silt+clay (ASC) and the aggregated clay (AC) indices. Similarly among the macro (WSA > 0.25 mm) tests the single sieve percent water stable aggregates (S%) reflected such differences more than the water stability index (WSI) and the mean-weight diameter of water-stable aggregates (MWD). The WSA < 0.25 mm and S% indices were also the easiest to determine and the most reproducible. The ASC index correlated positively whereas the WSA < 0.25 mm negatively with all the macro indices. Differences in the plasticity index (PI) and the (sand+silt)/clay ratios (K) of these soils were most clearly reflected by the AC index. This suggests that these physical parameters are closely related to structural stability at this micro level.