A tsetse survey to assess the risk of trypanosomosis to livestock was conducted in the south wesern Rift Valley of Ethiopia. The Challenge was estimated at the Product of tsetse relative density. Infection rate and the Proportion of bloodmeal taken by tsetse flies from Livestock.rnA total of 2900 km2 of the study area was divided into 100 km2 Monitoriag grids and Stratified in three strata according to altitude as lowland (0-1600 m alitiude) mid-altitude (1601-2000 m altitude) and highland (above 2000 m altitude.) The survey was done twice according to this stratification first during the dry spell (April-mid May) and second in late June-July 1999 (wet).rnTwentyfive NG2U-traps were deployed in each grid. Few biconical traps were deloyed along the rivers to monitor riverine species. Traps were removed after three days.and the flies were counted. Identified and sexed.Relative densiy was calculated as the number of flies caught per trap per day. Tsetse infection rates were determined by dissection method. Aging of flies was done mainly by wingfray method. Bloodmeal identification was based on enzyme-linked immunosorbent assay (ELISA) using host specific antisera.rnTwo tsetse species were detected,Glossina Pallidipes as the Predominant Species which was detected up to 1931 m above sea level and Glossina fussipes. Detected up to 1710 m altitude During the dry spell.the relative density of G pallidipes was 0.6 flies/trap/day and 0.06 flies/trap /day for G fuscipes. During the wet spell.these figures rose to 2.4 and 0.1 for G.Pallidipes and G.fuscipes respectively.The lowland areas recorded the highest density for both species. There were no flies caught in the highland area.rnAbout 915 G.pallidipes and 652 G.fuscipes were dissected for infection rate estimation. All flies were from lowland areas. For G.pallidipes. The overall infection rate was found to be 7.9%. There was a significaiontly higher infection rate in females (9.4%) than in males (4.1%) (x2=6.42.P < 0.05).The infection rate was also significantly higher in the wet (9.5%) than it was in the dry months (5.6%).(x2=4.57,P