A biological comparison based on differences in the starting material for composting processes was made. Mesophilic aerobic microbiota, fungi, actinomycetes and hemicellulolytic microorganisms reached significantly higher levels in the MSW final product. The population of cellulolytic microorganisms did not show a clear trend, although it was more numerous in the HW piles. Counts for N(2)-fixing and ammonifying bacteria were significantly higher in the SS pile at the early stages of the process, while populations tended to become equal as time progressed. The lowest populations were detected for nitrifying bacteria, with higher but not always significant levels for the SS pile. beta-Glucosidase and phosphatase activities were higher in the SS pile at the early stages. Protease reached its maximum activity during the bio-oxidative phase and final stages in the HW and MSW piles, respectively. Dehydrogenase activity, with an occasional high level for the MSW at the beginning of the process, was almost inexistent since the end of the bio-oxidative phase. On the contrary, urease showed higher levels at the final stage of the process, with the MSW pile showing the greatest levels most of the time. According to these results, the nature of the starting material causes differences in biological parameters.