Tropical theileriosis, caused by the haemoprotozoan parasite Theileria annulata, is one of the major threats to cattle health in Tunisia. The aim of the study described in this thesis was to assess the extent of diversity —T. annulata parasite populations from within a single country in order to provide a better understanding of the Semiology of the disease and biology of the parasite. The work also provided useful background information raccine development studies in Tunisia. Different T. annulata stocks, isolated from different bioclimatic zones in Tunisia were characterised using monoclonal antibodies (MAbs), isoenzyme and DNA analyses and compared with each other and with a number of recognised laboratory stocks from other theileriosis endemic areas of the world