Exposure to microorganisms, including thermophilic actinomycetes and fungal spores, and to airborne dust produced during compost production and mushroom picking may cause work-related respiratory symptoms. Previous studies have implicatedThermoactinomyces vulgarisandFaenia rectivirgula, the aetiological agents in farmer's lung disease, as causes of these symptoms but these species have been rare in aerobiological studies of mushroom farms. As part of a study of the respiratory health of the exposed workers, we carried out an aerobiological survey of all the stages of commercial mushroom production. Samples of viable airborne microorganisms were collected at a farm from eight locations on two occasions using Andersen cascade impactors. Large numbers of airborne thermophilic actinomycetes, yielding > 106colony-forming units (cfu) m−3air sampled, were associated with compost handling. These were predominantlyThermomonosporaspp., whileThermoactinomycesspp. andFaenia rectivirgulawere few. Because the compost was largely undisturbed, few airborne actinomycete spores were found in mushroom growing houses, but concentrations of fungal spores exceeded 105cfu m−3when mushrooms were being harvested. Most werePenicilliumspp. andAspergillus fumigatusbutPeziza ostracodermaandTrichodermaspp. were also isolated. Workers are thus exposed to a wide range of airborne microorganisms, but the role of many of these in mushroom workers' respiratory symptoms is not yet fully understood.