In 1917 Montgomery described a tick-borne gastro-enteritis of sheep and goats for which the name Nairobi sheep disease has passed into general use. In a comprehensive paper dealing with his work at Kabete he was able to show that the causal agent was a filtrable virus, transmitted by Rhipicephalus appendiculatus, the ordinary brown tick and common vector of East Coast fever in cattle. In spite of Montgomery's careful description of the lesions, of his record of failure to infect cattle, and of his successful filtration experiments, there has until recently been considerable confusion between this disease and heartwater, and opinions have been expressed that Montgomery was in all probability working with a strain of heartwater. During the course of certain investiga tions on heartwater the opportunity was taken to compare the two diseases and to confirm and extend Montgomery's observations.