The reduction of helminth eggs, fecal coliforms and somatic coliphages present in sewage sludge after treatments and abatement by application to soil was determined. Traditional stabilization processes produced small changes in the concentrations of the parameters studied. In contrast, thermal treatments and liming produced dramatic reductions. Fecal coliforms were the most affected by both types of treatments; somatic coliphages showed some persistence after 30 minutes at 60°C; and both somatic coliphages and helminth eggs showed some persistence to storage in quick lime. However, both treatments supplied biosolid suitable for unrestricted application in agriculture. Abatement in soils in the climatic conditions tested (mild to cold temperatures and high relative humidity) was slow and took several months to reach the background levels. These results suggest that environmental conditions (temperature and humidity) play the main role in inactivating the microorganisms, since abatement was similar in different soil types. The extended permanence of pathogens and microbial indicators in soil after the application of treated sludges indicates that, in the normal weather conditions of the areas where the study was performed and the amounts of sludges applied, contaminant microbes are not easily mobilized from the complex matrixes that constitute the treated sludges and that consequently in normal conditions their release as diffuse pollution is of lesser importance.