Actinomycetes, especially thermophilic species, are well known components of the microflora of composts. Composts for mushroom cultivation, prepared from animal manures and straw, have been most studied but actinomycetes may also colonise household and green waste composts. Actinomycetes are Gram-positive bacteria that mostly possess a mycelium. Thermoactinomyces spp. also produce a mycelium and have been generally considered with the actinomycetes but they produce endospores and are closely related to Bacillus spp. Many actinomycete species produce spores which easily become airborne in large numbers when the substrate is disturbed and some cause different forms of extrinsic allergic alveolitis. Composting for mushroom cultivation takes place in two phases, the first in windrows with large water contents and the second in humidified tunnels heated to 55-60°C. Actinomycetes, particularly white Thermomonospora spp., Thermomonospora chromogena and Microtetraspora spp., develop abundantly during the second phase and many spores are released during spawning. However, no one species has been implicated in mushroom worker’s lung. A similar microflora occurs in composts made from household waste but those from green waste often have microfloras dominated by Streptomyces spp., especially during the cooler winter months when windrow temperatures may be lower than in summer. Sewage composts are also rich sources of actinomycetes which may include Nocardia and Promicromonospora spp. Actinomycete development is dependent on aerobic conditions, temperature and water content although the interrelationships of these factors and the occurrence of different taxa have not been closely studied in composts