Abstract: Peripheral blood samples were collected from 162 horses and donkeys in various regions of Urmia area-Iran and the presence of microfilariae was evaluated by the Knott method. On the basis of morphological identification, 52 of 162 (32.1%) were infected with Setaria equina (40 cases), Parafilaria multipapilosa (6 cases) and Onchocerca reticulata (6 cases) and the infection with Setaria was confirmed in a case by necropsy. The number of microfilarae was between 1 and 935 larvae in 1 mL of blood. There was correlation between the time of sampling, age or sex and the prevalence of infection. The prevalence was higher in females (43 vs 31%)and in age group 7-16 vs 1-6 year (34.3 vs 32.3%). There was a significant association between the prevalence of microfilaraemia and the climate condition, thus it was higher between late spring and early fall because of more vector abundance and activity (50 vs 3.2%).