Background: The aim of the study was to assess the occupational exposure to mesophilic microorganisms associated with commercial processing of compost for mushroom production. Material and Methods: The air samples for microbiological analysis were collected directly on Petri dishes with Malt Extract Agar medium using Burkard Air Sampler. The quantitative and qualitative identifi cation of mesophilic bacteria and mesophilic fungi were performed using the standard microbiological procedures. Results: Considerable variation in exposure to bioaerosols within the sectors of compost production were observed. The highest level of mesophilic bacteria in the air (4.17×104 cfu/m³) was measured during the mixing of raw materials with water and loading of the fi nal compost piles to the pasteurization tunnels (3.54×104 cfu/m³). The highest concentration of mesophilic fungi was found in the air samples collected during the manual unrolling of straw (1.62×104 cfu/m³) and dosing of mycelium to the compost (1.15×104 cfu/m³). Among 33 identified bacteria and fungi species, Pasteurella sp, Proteus mirabilis, Streptomyces sp, Corynebacterium sp and Aspergillus fumigatus create the potential risk for health of exposed workers. Conclusions: The presence of microorganisms found in the air creats the potential risk for human health, therefore the use of personal equipment, protecting the respiratory tract and skin of exposed workers is strongly recommended. Med Pr 2008;59(5):373-379