Technical Abstract: Long-term tillage and cropping may influence biological attributes responsible for semi-arid soil aggregation in Montana, USA. Aggregate stability, glomalin, basidiomycete fungi, uronic acids, total organic C (TOC) and total N (TN) at 0-5 cm soil depth from 1991 to 2003 were evaluated in different aggregate-size classes (4.75-2.00; 2.00-1.00; 1.00-0.50; and 0.50-0.25 mm) for the following treatments: 1) 12th year of fallow phase after 11 years of conventional- and no-tilled spring wheat-fallow (CTF and NTF), 2) 12th year of lentil phase after 11 years of conventional- and no-tilled spring wheat-lentil (CTL and NTL), 3) 12 years no-tilled continuous spring wheat (NTCW), and 4) 16 years undisturbed pasture (P). No-tilled continuous wheat may increase soil aggregation by increasing the biological attributes tested compared with conventional-tilled wheat-fallow. Similarly, grassland soils may increase soil aggregation by increasing these attributes compared to cropland soils. Aggregate stability was increased in conventional till compared with no-till and in grassland compared to cropland in middle aggregate size classes (2.00-1.00; 1.00-0.50 mm). In general, adding lentil in rotation with wheat did not influence aggregate stability compared to fallow-wheat after 12 years, however, leaving a moderate amount of lentil residue on the soil surface in no-tilled lentil-wheat (NTL) seems to be a better alternative than fallow (CTF and NTF) or lentil-wheat under tillage (CTL) to minimize N mineralization from the legume residue and still control erosion by increasing soil stability.