Microbiological characteristics of sewage sludge from a mechanical-and-biological sewage treatment plant composted in controlled conditions with straw and sawdust are presented. Prepared composts were placed in four bioreactors with airflow of 4 l air·min^-1. In bioreactor K1, K2 and K3 the composted mass consisted of 65% sewage sludge (K1–sewage sludge 1, K2 – sludge 2, K3 – sludge 3) + 30% sawdust + 5% straw; while in bioreactor K4 the proportion was: 45% sludge 2 + 50% sawdust + 5% straw. Compost samples were taken from all chambers at the same time, depending on actual temperature. Microbiological analyses consisted in the determination (by plate method) on selective media of the numbers of mesophilic and thermophilic bacteria, fungi and pathogenic bacteria Salmonella sp. Clostridium perfringens and Enterobacteriaceae. Furthermore, in the experiment, the activity levels of dehydrogenase were determined using 1% triphenyltetrazole chloride as substratum. Studies have indicated that the composting process caused a decrease of the number of fungi and pathogenic bacteria from Enterobacteriaceae family and Clostridium perfringens in all composted matters, as well as an increase in the number of thermophilic bacteria. Changes in the number of mesophilic bacteria depended on the compost type. In composts K1 and K2, the composting process caused an increased proliferation of cells, while in the composts K3 and K4 the number of mesophilic bacteria decreased. On the basis of the obtained results, it was also found that in the majority of analysis terms, the lowest activity of dehydrogenases occurred in compost K3, while their level of its activity, in the majority of the studied composts, correlated most intensively with the number of thermophilic bacteria.