Microbial populations and enzyme activities in a soil amended with sewage sludge in a long-term field experiment were studied in relation to the decomposition of organic matter Two kinds of sewage sludges were applied to the soil at rates of 50 and 100 tons/ha/year for 8 years. The percentage of humic acids increased in the soil with the application of sewage sludges. The enzyme activities exhibited a high correlation coefficient with the number of microorganisms and the content of humic substances. The results suggested that the sewage-sludge application increased microbial activity in the soil, contributing to the gradual decomposition of organic matter. Copyright (C) 1996 Elsevier Science Ltd.