Typing of Mycobacterium avium strains obtained in a study of endemic tuberculosis in a Wildfowl Reserve permitted the recognition of two separate infected groups. The main infection was in Anatidae and was due to M. avium, type 1; the other was in chickens used for incubation and brooding and the predominance in it of type 2 agreed with normal experience of birds, pigs and cattle in Britain. Many of the strains isolated from the Anatidae were aberrant and methods used to investigate these are described; two of the strains may belong to a new type. Birds which died from other causes, usually trauma, often had subclinical tuberculosis and 5% of the samples of mud and soil examined yielded M. avium.