The physical-chemical characteristics, (pH, Eh, S2−, TOC, NH4+, particulate Kjeldahl nitrogen (PKN), total phosphorus (TP) and PO43−.) of the sludge accumulated in the first of a series of wastewater stabilization ponds nave been studied in two vertical profiles in order to characterize the effects of the maturation of the sludge. The effects on the concentration of faecal indicator organisms and salmonella have also been determined. The maturation leads to an increased density of the sludge and to the removal of about 80% of the TOC, 70% of PN and of 75% of TP initially present in the fresh sludge. The concentrations of sulfate reducing bacteria do not change and those of the total coliforms, the faecal coliforms and the faecal streptococci are only moderately modified, as the maximum decrease is 4 log. Salmonellae only are found in the superficial sediment layer, which might indicate their rapid inactivation.