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Ehrlichia phagocytophila (Genus Ehrlichia, Order Rickettsiales) is the pathogen responsible for Tick-borne fever, a disease of high morbidity in susceptible ruminants. These bacteria appear to be almost identical at serological and molecular level to granulocytic Ehrlichia species recently diagnosed in humans, dogs and horses of Europe and the United States. A molecular description of different isolates of the pathogen is given. Samples were derived from wild and domestic vertebrate hosts from Europe, including the UK where Tick-borne fever is endemic. Molecular characterisation of a fragment from the groE operon gene showed higher nucleotide variation than at 16S rDNA level. Human and equine isolates from Europe differed from North American samples, which in 16S sequence appeared to be identical. Thus, emerging cases of granulocytic ehrlichiosis in Europe are likely to be associated with strain variants of E. phagocytophila. Further differences were also found between ruminant and non-ruminant samples of granulocytic Ehrlichia from Europe. Genomic analysis of less conserved genes appears necessary to provide more useful phylogenetic information that will help to clarify the relationship between closely related bacterial species. Populations of the vector tick, Ixodes ricinus, were sampled and analysed to determine the prevalence of infection and their role in the epidemiology of the disease. Studies indicated a low infection prevalence that seems, however, enough to maintain the pathogen in nature. The prevalence varied according to widespread sites across Britain but it was always lower than expected from information in the literature. Attempts to determine the efficiency of latent infection in sheep to transmit Ehrlichia to ticks were unsuccessful. A seroepidemiological survey was undertaken using IFAT and involving samples from suspected vertebrate reservoirs of infection such as dogs, cats, horses, and deer in order to determine if those species were exposed to the pathogen and the range of hosts for the bacteria in widespread sites in Britain. The results suggested high rates of exposure in dogs from rural areas and wild roe deer. Cats also showed a high seroprevalence indicating the three vertebrate hosts were exposed to E. phagocytophila and mounted an immune response towards the pathogen.