Relationship between water-stable aggregation and microbial activity as measured by CO2 evolution was studied for mixtures of sand and bentonite clay with additions of ground alfalfa and corn stover, untreated and hot-water-extracted. In 3000 hrs. of incubation, 2 peaks of aggregation occur as the result of accumulation of decomposition products and metabolic wastes from microbial action. The initial peak, shown only when readily decomposable organic matter was present, occurred in 6 days; a 2d peak for all treatments, in 34. The aggregates disintegrate after each peak.1 The 2 maximums of aggregation are not additive.