A cross-sectional study was conducted to determine the prevalence and risk factors for ectoparasites infestation in sheep in three agro-ecological zones in central Oromia, Ethiopia, from October 2009 to April 2010. The study revealed that 637 (48.1%) of the 1325 sheep examined were infested with one or more ectoparasites. The ectoparasites identified were <em>Bovicola ovis</em> (27.2%), <em>Melophagus ovinus</em> (16.4%), <em>Ctenocephalides</em> sp. (2.3%), <em>Linognathus africanus</em> (1.2%), <em>Linognathus ovillus </em> (0.3%),<em> Sarcoptes</em> sp. (1.2%), <em>Amblyomma variegatum</em> (4.4%), <em>Rhipicephalus evertsi evertsi</em> (1.9%), <em>Rhipicephalus pravus</em> (1.9%), <em>Rhipicephalus</em> (<em>Boophilus</em>) <em>decoloratus</em> (1.1%), <em>Rhipicephalus sanguineus</em> (0.9%), <em>Rhipicephalus praetextatus</em> (1.1%) and <em>Hyalomma truncatum</em> (1.6%). Statistically significant difference was observed in prevalence of <em>B. ovis</em> amongst study agroecological zones: highland 36.6%, midland 20.9% and lowland 14.0%. Significantly higher prevalence was recorded in highland agroecological zone. A significantly (OR = 0.041, <em>p</em> < 0.001) higher prevalence of <em>M. ovinus</em> in the highland (31.7%) than in both the lowland (0%) and midland (1.9%) was observed. The risk of tick infestation in the lowland and midland was 9.883 times and 13.988 times higher than the risk in the highland, respectively. A significantly higher prevalence of <em>Ctenocephalides</em> species was encountered in both the lowland (OR = 4.738, <em>p</em> = 0.011) and midland (OR = 8.078, <em>p</em> = 0.000) than in the highland agro-ecological zone. However, a significant difference (<em>p</em> = 0.191) amongst agro-ecological zones was not found for the prevalence of <em>Linognathus</em> and <em>Sarcoptes </em>species. Statistically significant variation (<em>p</em> > 0.05) was never recorded in the prevalence of all the identified species of ectoparasites between male and female sheep hosts. However, a significantly (<em>p</em> = 0.006) higher prevalence of<em> B. ovis</em> was recorded between young and adult sheep. The risk of <em>B. ovis</em> infestation was 1.45 times higher in young than the adult sheep. Furthermore, a significantly (<em>p</em> < 0.001) higher prevalence of <em>M. ovinus</em>, <em>B. ovis</em> and <em>Sarcoptes</em> sp. was found between sheep with poor and a good body condition. The ever increasing threat of ectoparasites on overall sheep productivity and tanning industry in Ethiopia warrants urgent control intervention. Further studies on the role of ectoparasites in transmission of diseases to sheep, zoonotic importance, comparative prevalence and load, and the importance of sheep as alternative hosts in different agroecological zones, breeds and management systems in Ethiopia are recommended so as to design applicable control programme in the country.