The effects of the application as fertilizer during ten years of two sewage sludges (aerobically and anaerobically digested, at rates of 400, 800, and 1200 kg of N/ha yr), on the aggregate stability and contents of related organic matter components, microbial biomass and levels of five enzymatic activities (alkaline phosphomonoesterase, phosphodiesterase, urease, arylsulphatase and dehydrogenase) were investigated. The application of both sludges at mid and high rates gave rise to significant increases of organic matter, humified substances and humic acids, but no effects on carbohydrates, microbial gums and aggregate stability were observed. As for biological activity in soils, the high Variability of data led to a general absence of statistical significance despite the large differences between treatments observed. Significant increases of phosphodiesterase activity were nevertheless produced by the high rate of aerobic sludge and the mid and high rates of the anaerobic sludge. (C) 2001 Elsevier Science Ltd. All rights reserved.