The aim of this work has been to determine the effect of sterilized and non-sterilized, aerobically or anaerobically digested sewage sludges on urease and phosphatase aetivities, on populations of nitrite oxidizers and on some chemical properties in laboratory conditions and for long incubation periods. Both urease an phosphatase activities were affected when anaerobic sludges were added to the soil. The inhibitory effects on both enzyme activities attributed to the presence of heavy metals were probably masked by the additional source of organic matter both with aerobic and anaerobic sludges. The level of the NO,- oxidizer populations was higher in the aerobic than in the anaerobic sludge amended soil. Ammonia-N concentration were increased when the soil received sludges reflecting a higher mineralization rate probably due to a stimulation of the indigeneous ammonia producing microorganisms since differences between non-sterilizcd and sterilized treatments were minimal or absent. Nitrate-N concentrations increased in amended soils and care must be taken to restrict as much as possible the downward movement of nitrate in order to minimize ground-water pollution.