The present study was undertaken to demonstrate the effects of fly-ash addition to sludges on nitrogen content and urease activity of soils. Wastewater sludges added with varying doses of fly-ash (40, 80 and 120%, on dry weight basis) were amended to a clay soil, and the variations in ammonium, nitrate and total nitrogen contents and urease activities were monitored during incubation at 28 degrees C for 360 days. Co-application of fly-ash and wastewater sludge, especially at the dose of 120 % fly-ash, caused an initial decrease in inorganic nitrogen and urease activity levels in contradiction to sludge-only application. However, after the first 3 months of incubation, this adverse effect almost disappeared. Application of fly-ash/sludge mixture to soil with appropriate dosages (< 120%) may not affect the microbiological processes occurring in soil system.