Onchocerca gutturosa is prevalent in Sudanese cattle, and aspects of its behaviour, transmission, and morphology were studied and where possible comparisons have been made with the same species in U. K. A vector, Culicoides kingi, was discovered and the complete development of 0. gutturosa was studied. The adult worms were found to inhabit the'ligamentum . nuchae and stifle joint. None were found in the gastrosplenic ligament. All adults and microfilariae found were 0. gutturosa (Neumann, 1910). The microfilarial distributions in the skin around the hump region in Sudanese cattle contrasted sharply with those'reported in English cattle, presumably an adaptation to the biting habits of C. kingi in the Sudan. Pathological effects of 0. gutturosa in the ligamentum nuchae of cows were minor. Distinct morphological differences were found between microfilariae in the uterus of adult worms and those in the dermis of cows. Those emerging from the uterus of adult females did not develop in the vector. There were also two forms of microfilariae in the uterus of adult females which appeared to represent two stages of maturity. In the skin of Sudanese cattle microfilariae of 0. gutturosa were found in the"same region as microfilariae of 0. armillata and of 0. raillieti in donkeys. Microfilariae of 0. gutturosa could be readily distinguished from the other species by differences in size, number of nuclei anterior to the nerve ring and in arrangement of the caudal and cepkhalic nuclei. C. kingi was discovered to be a vector at a dairy farm near Khartoum. Although the midge showed seasonal changes in abundance, there was no clear seasonal variation in number of microfilariae found in the dermis of Sudanese cattle. However, maximum numbers of microfilariae coincided with maximum activity of C. kingi. Biting activity of the midge showed 2 daily peaks. Ninety five per cent of the attacking C. 'kingi landed on the back of the cow and 25% were successful in obtaining a blood'meal. The attractiveness of the hump surface was related to the lack of-hair and less host disturbance. Complete adult 0. gutturosa were obtained from Sudanese cattle using a digestion technique. Living male and female worms were transplanted into the peritoneal cavity of different strains of mice; some survived up to 133 days. The behaviour of 0. gutturosa microfilariae in the Nile rat and in mice was studied for a screening model. Chemotherapy was also carried out in naturally infected cattle.