Studies on the pathogenicity of Histophilus ovis in sheep were carried out over a seven year period from pathological material received at the District Veterinary Laboratory, Armidale. Four distinct disease syndromes were encountered from which Histophilus ovis was isolated in pure culture and accepted as the causative agent. Mastitis in ewes caused by Histophilus ovis appeared as an extremely rare condition but was acute in nature and led to rapid loss of gland function, generalised toxaemia and death in untreated animals. Isolation and identification of the pathogen was essential for differential diagnosis and the correct choice of antibiotic for therapeutic use. Epididymitis in rams due to Histophilus ovis was sporadic in occurrence with only small numbers of rams in a flock being affected. Lesions impaired fertility but as they were usually unilateral, complete sterility was rare. The disease wasithronic in nature but differential bacteriological diagnosis was essential to distinguish the syndrome from other causes of epididymitis so that effective control measures could be undertaken. Polyarthritis in lambs resulting from localisation of Histophilus ovis in the joints was acute in nature and resulted in exteme lameness and debility in untreated sheep. Lesions were non— suppurative in nature and differential diagnosis was assisted by the lack of any association of lameness with any managerial practices. Abortion in ewes caused by Histophilus ovis was also extremely rare in occurrence but resulted from acute haemorrhagic vi placentitis. Affected ewes soon recovered and did not appear to harbour infection into the next lambing season. However, the one outbreak investigated severely depressed lambing percentages when it occurred. Isolates obtained from these four disease syndromes allowed for detailed bacteriological investigations. The organism described as Histophilus ovis is closely related to certain members of the Family Brucellaceae but various tests used routinely in diagnostic bacteriological laboratories allowed for its differentiation. Strains of the organism isolated from the different disease syndromes and from different geographical areas were found to give identical results to a wide range of characterization tests. I Serological studies on the organism showed the isolates to be antigenically homogeneous and also that they were closely related, if not identical, to the organism previously described as Actinobacillus seminis.