Technical Abstract: Tillage may influence the proportions of microorganisms involved in soil aggregation. We evaluated the effects of no till (NT) and conventional tillage (CT, tillage depth about 7 cm) continuous spring wheat system on bacterial communities predominant in microaggregates (0.25-0.05 mm) and on saprophytic basidiomycete fungi in aggregate-size classes (4.75-2.00, 2.00-0.25, and 0.25-0.05 mm) at 0-20 cm depth of a Williams loam (fine-loamy, mixed, Typic Argiborolls) in semiarid eastern Montana, USA. Enzyme-linked immunosorbent assay used to quantify basidiomycete fungi, was higher in NT than in CT in 4.75-2.00 mm size class in 2007 and higher in all size classes and years at 0-5 cm depth, but was not different between tillage, years, and aggregate-size classes at 5-20 cm. The predominant bacteria from microaggregates were subjected to a soil sedimentation assay to determine their soil binding capability. The proportion of isolates which can function as soil aggregators was higher in NT than in CT at 0-5 cm but was not different at 5-20 cm. Our results provide a first insight into the beneficial effects of dryland NT compared to CT in probably reducing soil disturbance and residue incorporation and enriching the proportion of microorganisms responsible for aggregation, especially at the surface soil.