One thousand two hundred and sixty seven isolates of Bacteroides nodosus from 292 sheep in 58 flocks were examined. Of these, 1260 could be classified by slide agglutination into 8 serogroups designated A to H. Up to 6 serogroups were detected in individual flocks, with up to 4 serogroups being detected in a single foot. Of the 292 sheep examined, 38 (13%) carried mixed serogroup infections. Determination of the range of serological types infecting a flock frequently required the examination of a number of isolates from each of a number of sheep. Cross-tube agglutination tests carried out on 44 isolates and their antiserums indicated that members of some serogroups could be divisible into subgroups or serotypes. These results suggested that 16 or more serotypes of B. nodosus might exist. The nature of the antigens responsible for both slide and tube agglutination reactions needs to be determined.

Sheep exposed to uncontrolled outbreaks of footrot were observed on one farm for 3 successive years and on another for one year. Observations were made from mid-winter to early summer. In each outbreak about 70% or more of sheep exposed to footrot acquired the disease. Natural remission occurred in each outbreak and some cases healed while new cases continued to develop. A feature of all outbreaks observed was that a much higher proportion of late onset cases healed than cases which developed early in the outbreaks. By contrast, 90% of sheep already affected before spring had lesions which persisted into next summer. It is suggested that there is a spectrum of resistance to footrot among sheep. This spectrum includes animals which remain chronically infected and those that do not become infected at all. Intermediate levels of resistance are reflected firstly in the time taken by different groups of sheep, to become affected after exposure and secondly, to heal after becoming affected.