Trypanosomiasis, whether it affects an or domestic animals, continues to be one of the post baffling problems which has confronted medical and veterinary parasitologists. Surra is the most important of all the animal trypanososiases since it, has extremely wide geographical distribution and affects, a wide variety of mammals. The causative organism T : noaoma (Trypenozoon) evens i (Tev nsi) was the first trypanosome to have been recorded from a mammal, but many fundamental aspects of the parasite such as morphology, taxonomy and phylogeny are not parlectly understood. Hitherto, a cycle of development has not been demonstrated in an invert ate vector mnd the parasite is believed to be transmitted non-cyclically. Only a few attempts have been made to culture the parasite in bloodagar medium. Comparatively little work has been carried out on the immunology of T. evansi. Certain Immunological tests, e.g., indirect haemagglutination, agglutination, precipitation and fluorescent antibody tests have been demonstrated in experimental T. evansi infections, but an Important immunological aspect, antigenic variation, has not yet been investigated in this species. The present study is concerned with the establishment of antigenic variation of T. evansi in the mouse host and subsequent comparison of the antigenic types of the first natural relapse populations. The findings on antigenic variation in to evansi have been compared with similar studies in other species of the subgenus Trypanozoon. Furthermore, the importance of the 'recurrent antigenic type', which was observed In these fnvestigations, has been evaluated with respect to the immunlogical classification of salivariaa trypanosoms. Pleomorphisa was experimentally induced in an old monomorphic laboratory strain of T. evansi and attempts were made to culture both monomorphic and experimentally developed pleomorphic strains. Subsequently, the transmission of monomorphic and experimentally developed plaomorphic strains of T. evansi employing Glossina moraitans, Stomoxys calcitrans and Ornithodoros moubata was investigated. In view of the similarity between pleomorphic strains of T. evansi and those of trypanosoma (Trypanosoon) brucei, the systematic position of T. evansi has been discussed. The current concept on the phylogeny of T. evansi is critically reviewed and a different opinion on the subject is presented. During these studies the effect of environmental temperature on T. evansi infection was also studied and clone populations were set up to ctaractariss 3 strains of T. evansi. The thesis embodies observations on each of these aspects and the relevant literature, together with the present state of knowledge concerning T. evansi , has been reviewed.