Actinomycetes are a specific group as bacteria. Morphologically they resemble fungi because of their elongated cells that branch into filaments or hyphae.
During the process of composting mainly thermophilic (adapted to high temperatures) and thermotolerant actinomycetes are responsible for decomposition of the organic matter at elevated temperatures. In the initial phase of composting the intensive increase of microbial activity leads to a self heating of the organic material. High temperatures in composting help to kill viruses, pathogenic bacteria, e.g. coliforms, and weed seeds present. Agarplate with actinomycetes Actinomycetes live predominantly aerobically, i.e. they need oxygen for their metabolism. The compost material should therefore be well aerated. Generally, actinomycetes grow on fresh substrates more slowly than other bacteria and fungi. During the composting process the actinomycetes degrade natural substances such as chitin or cellulose.
Natural habitats of thermophilic actinomycetes are silos, corn mills, air conditioning systems and closed stables.

Some thermophilic and thermotolerant actinomycetes are found to be responsible for allergic symptoms in the respiratory tract (e.g. "extrinsic allergic alveolitis", EAA).