The distribution of pathogenic anaerobes in the environment and the relationship with diseases in animals are discussed. A distinction between the spore-bearing anaerobes (clostridia) and the Gram-negative non-spore-forming anaerobes is necessary. The main habitat of clostridia is the soil but they are also found in dust, sewage, rivers, lakes, sea water, milk, vegetables, fresh meat, fish, insects and the intestinal tract. The Gram-negative non-spore-forming anaerobic bacteria are also widely distributed among animals, principally on mucous membranes of the alimentary tract. After a general introduction and a section on the isolation of anaerobes, the various diseases caused by clostridia (botulism, tetanus, blackleg, malignant oedema, infectious necrotic hepatitis, enterotoxaemia and gas gangrene) and Gram-negative anaerobes (infections due to Fusobacterium and Bacteroides spp., such as diphtheria, footrot, etc.) are discussed. In particular, information concerning the reservoir of the causative agent and the mode of transmission is presented.