Fourteen cases of lymphosarcoma occurred in a small experimental flock of sheep in Rhodesia. There appeared to be horizontal spread and the ages of the affected animals varied from one to more than 10 years. Thirteen of the cases so far diagnosed were leucaemic with a high lymphocyte count. Eleven of the sheep demonstrated multicentric lesions and two alimentary mesenteric; the remaining animal is still alive. Transmission attempts in four neonates resulted in one showing a high white cell count after 25 months. Electron microscope studies failed to demonstrate C-type virus-like particles. This appears to be the first recorded example of an outbreak of lymphosarcoma in a flock of sheep in Southern Africa and the descriptive term "enzootic leucosis" has been adopted.