The burden of ticks in semi-arid lands of Ethiopia is not as pronounced as in some more humid areas of the continent. Nevertheless, the increasing recourse to chemicals smuggled by illegal traders has led to serious problems, including poisoning of humans and animals, discontinuous and irrational treatment regimens, tick-resistance to acaricidal products, loss of traditional knowledge and weakening of social structures. In order to encourage a resumption of the long-established ethnoveterinary practices, a survey on plants locally used in tick control was undertaken, and plant species used in other parts of the continent for the same purpose were considered. On these bases, 28 plant species or varieties were collected in the study area: Acacia seyal var. seyal, Adenium somalense, Aloe calidophila, Aloe parvidens, Azadirachta indica, Boscia angustifolia, Calotropis procera, Calpurnia aurea, Cissus quadrangularis, Commiphora erythraea, Cordia africana, Croton macrostachys, Croton megalocarpus, Datura stramonium, Euphorbia candelabrum, Euphorbia tirucalli, Ficus sycomorus, Ficus thonningii, Lantana camara, Maerua triphylla, Ocimum suave, two varieties of Ricinus communis (one with green fruits and another with red ones), Solanum incanum, Solanum somalense, Sterculia rhynchocarpa, Tagetes minuta and Vernonia amygdalina. In general, leaves were collected and used. However, due to the scarcity of foliar material, the whole plant of T. minuta and O. suave, the whole stem of A. somalense and C. quadrangularis, the branches of E. candelabrum and E. tirucalli, the bark in the case of A. seyal, C. erythraea and S. rhynchocarpa, were examined. After drying and grinding, the plant material was extracted with hexane and acetone, and made up to different concentrations to test the relevant repellent and toxic properties on adult Rhipicephalus pulchellus unfed ticks. For every bioassay, four replications, each using ten ticks, were performed. For the repellency bioassays, ticks were placed on a rectangular polystyrene platform stuck in a plastic basin and surrounded by water, in order to prevent them from moving away. Two glass rods, each provided with filter paper at the top and at the base, were inserted at opposite edges of the platform. The two filter papers of one rod were impregnated with the testing solution (i.e. solvent plus extract) at different concentrations while those of the other rod were treated with the pertinent extractant (hexane or acetone). Because of their inherent tendency to climb, most of the ticks settled onto the rods (mainly at the top), and their distribution was different depending on the repellency capacity of the extracts. The relevant data were then converted into repellency indexes using the formula [(Nc - Nt)/(Nc + Nt)] x 100, where Nc refers to the number of ticks on the control rod and Nt to the number of ticks on the test rod (Lwande et al., 1999; Pascual-Villalobos and Robledo, 1998). For the toxicity bioassays, 1 ìl of the extract at different concentrations was placed onto each tick and the mortality or weakening ratio was recorded after 24 hours. Because of the intrinsic toxicity of hexane, only acetone extracts were used for these assays. Due to the efficacy in extracting volatile compounds, hexane extracts had, for 24 plant species, better repellent properties than acetone extracts. Moreover, at a concentration of 10%, four species had negative repellency indexes with hexane extracts and five with acetone ones. At such concentration, these extracts therefore seemed to attract the ticks rather than repel them. At a concentration of 10%, thirteen hexane and five acetone extracts had repellency indexes > 50. At a concentration of 5%, only five hexane extracts and no acetone ones exceeded this value. Finally, only one species had a repellency index > 50 with the hexane extract at a 1% concentration. The plants showing good repellency indexes with at least one of the two solvents were A. calidophila, C. quadrangularis, C. erythraea, C. macrostachys, C. megalocarpus, D. stramonium, L. camara, M. triphylla, O. suave, the two varieties of R. communis and T. minuta. Amomg them, from a practical point of view, it is suitable to concentrate on O. suave, T. minuta and, to a certain extent, A. calidophila. In fact, C. quadrangularis, C. erythraea, C. macrostachys, D. stramonium, M. triphylla and the two varieties of R. communis had good repellent properties using hexane extracts at 10%, but not at 5%. Because trees like C. erythraea, C. macrostachys, C. megalocarpus and M. triphylla are highly valuable in a very dry environment, an excessive exploitation can put them in danger. Since D. stramonium, L. camara and R. communis are toxic plants, their extracts can be a serious threat for both humans and animals. Furthermore, L. camara is one of the worst weeds in the world, making it very inappropriate for lands already subject to the problem of bush encroachment. For all these reasons, T. minuta and O. suave appear to be the most promising plants; moreover, they are very well known in Southern Ethiopia and occur widely all over the area. On the contrary, A. calidophila is limited to just some places and the cultivation of Aloe species needs special attention, so it is not very suitable for people with a nomadic lifestyle. Concerning the toxicity bioassays, C. aurea extracts yielded by far the best results. In fact, all the ticks used had severe movement impairment when put in contact with acetone extracts at the concentrations of 20% and 10%. At a 5% concentration, 85% of the ticks had the same symptoms. In a separate test, a 10% water extract had a similar effect on 30 ticks out of 40, demonstrating the ease of extraction and application of the active compounds. The plant is well known, mainly by the Borana pastoralists, and is resistant to drought. It is also well able to grow in overgrazed areas, and its cultivation does not require special skills. Some of the extracts of other species gave good or fair results in the toxicity bioassays but, apart from S. incanum, only at a very high concentration (20%). Further studies may include isolation and characterization of the active compounds from the best species, setting up of a suitable plan for livestock treatment, and organization of a production and distribution cycle of appropriate phytomedicines in the pertinent pastoral area. / Dissertation (MSc (Veterinary Science))--University of Pretoria, 2007. / Paraclinical Sciences / unrestricted