Research indicates that the presence of live roots, and reduced soil disturbance as associated with perennial crops (PR) can enhance soil aggregation. Therefore, we hypothesized that: i) PR and double-cropped small grains (SG) would enhance percent water-stable aggregates (WSA) more than summer annuals (SA) within and across four cropping systems, and ii) change in WSA would be greatest under SA>SG>PR from spring-summer, and summer-fall. Ten crops were studied in the four cropping systems of a conventionally-tilled 36-year old experiment: i) continuous corn (C-C), ii) corn-soybean (C-S), iii) 4 years of corn-4 years of alfalfa (4C-4A), and iv) corn-oats-wheat-2 years of redclover+timothy (C-O-W-2RT). Soils were collected at 0-15 cm in May, July, August, and November, 2005. The experimental design was RCBD with four replications. Data were analyzed with ANOVA & regression in SAS. Six variables representing tillage frequency and live-root-months in crops & systems were created to test which best predicted WSA. Of the six variables, proportion of rotation with live roots (PRL) explained the most variability in WSA. Stepwise regression selected only PRL, suggesting live-root-presence predicted WSA best. In ANOVA, the crops(cropping system)*date interaction was significant. Perennial and diverse systems (4C-4A, C-O-W-2RT) had higher WSA than annual cropping systems (C-C, C-S). Within cropping systems, however, PR didn’t always differ from SA. In spring and summer, the PR and the SG had higher WSA than the SA grown in the annual systems but similar to the SA grown in the PR systems. From spring to summer, the WSA increased linearly under the SA, but in a quadratic pattern under the SG and PR. Percent decrease of WSA from summer-fall was higher under SA than PR & SG within and across systems. Results indicate that cropping systems with live roots present for a high proportion of the rotation time promoted WSA.