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Nowadays, the problem of dealing with the organic fraction contained in urban solid waste (OFUSW) and the biosolids generated from wastewater treatment plants (WWTP) requires alternative solutions. Composting is a method that can transform these wastes for their use. The length of the process depends on initial substrates, particle size, arrangement of the pile, moisture, aeration, and active biological population. The objective of this study is to determine the effect of aeration on the growth of bacteria and actinomycetes in composting piles of 250 kg. The pile identified as P1 is made with clay (30%), lama (5%), biosolids of WWTP (32.5%) and OFUSW (32.5%); and the one identified as P2 is prepared with clay (30%), lama (5%) and biosolids (65%). During the process, moisture content in wet basis (Hbh), temperature, pH, and growth of bacteria and actinomycetes are monitored in both piles, before and after performing aeration. In the thermophilic phase, P1 reached 49.0 - 51.5 °C for five consecutive days, while P2 reached 45.2 - 48.4 °C for three days. In both piles, pH ranged from 6.78 to 8.75 and compost was obtained in 28 days. The moisture for P1 was 35-42%Hbh, and for P2 it was 32-35%Hbh. Aeration had a significant effect on microbial growth in the thermophilic phase because it altered the conditions of the microsystems, being lower in the case of actinomycetes than in the case of bacteria. [ABSTRACT FROM AUTHOR]