Microbial characterization of composting is of importance for the optimization of the process and the quality of the end product. The aim of this work was to follow microbial biomass C and N dynamics during the composting process of urban solid waste. Microbial biomass C (B(C)) ranged from 4.06 to 1 microg kg(-1) of dry compost from day 5 to day 62 and decreased to reach 0.44 mg kg(-1) in mature compost. Microbial biomass N (B(N)) showed the same trend as B(C), ranging from 1.472 to 0.443 microg kg(-1) of dry compost from day 5 to day 62. This behaviour is probably related to the decreasing availability of readily decomposable substrates with the ongoing of the process. B(C) and B(N) showed a strong correlation (r = 0.78). The dynamics of the B(C)/B(N) ratio, index of the chemical composition of the whole microbial population suggested a shift in the composition of microbial populations during the process from prevailing bacteria and actinomycetes to prevailing fungi.