A 3-year-old Maltese-cross dog presented with a 4-month history of chronic diarrhoea and inappetence. Poorly regenerative anaemia, leukocytosis and hypoproteinaemia were evident on several occasions. Biopsies of stomach, duodenum and colon revealed marked infiltration of mucosae by macrophages containing many acid-fast bacilli. Similar organisms were numerous in a faecal smear. Melaena, hematochezia and severe abdominal pain developed and were unresponsive to therapy. Following euthanasia and necropsy, histiocytic cells containing acid-fast bacilli were found throughout the gastrointestinal tract, mesenteric and peripheral lymph nodes, spleen, liver, kidney and lungs. The organism was identified as Mycobacterium avium by bacterial culture and polymerase chain reaction testing.