Pasteurellosis is an important cause of economic loss to the sheep industry. There are two distinct syndromes. The pneumonic form of the disease caused by P haemolytica biotype A occurs as pneumonia in flocks and sporadically in individual sheep. The septicaemic form, caused by P haemolytica biotype T is associated with hyper-acute disease and occurs most commonly in the autumn coinciding with the folding of hoggs on rape, turnips and improved pastures. The factors which predispose sheep to the different forms of the disease are poorly understood but recently it has been possible to reproduce pasteurella pneumonia experimentally.