The objectives of this study were to determine the rate of infestation with Hypoderma spp. Over the four season of the year and timing of the parasite life cycle in slaughtered ruminants in Urmia, Iran. After a preslaughter inspection of the animal, internal suface of the esophagus from its cranial to its distal end, spinal canal from cervical vertebrae to end of sacrum, internal surface of skin and the surface of body were examined for the presence of Hypoderma larvae in slaughtered cattle and buffalo. In sheep and goat only the internal surface of the skin and the surface of the body were examined. The results showed that Hypoderma bovis was the main Hypoderma spp. infesting cattle in that region. Initially, first instar larvae were seen in the epidural fat of the spine in mid-December, and they were found in the spine to early-March. The peak rate of infection in the spine was 47% found in January-February. The second and third instars were first found in subdermal tissue of back in February, then till July the larvae were seen under the skin of the back. The maximum rate of subdermal tissue infestation was 36% observed in mid-April The intensity and relative frequency of subdermal tissue infestation were more significant (p<0.001) in the cattle uncler 1.5 ages in comparison with older cattle. Hypoderma silenus was the only species parasitizing goats. The larvae were observed in the subdermal tissue of the back from mid-October to mid-March. The maximum rate of infection was 44% observed in mid- February. In buffalo and sheep, the rates of infestation were quite low, i.e. 0.8% and 0.36%, respectively. The results showed that the prevalence of infestation in the cattle and goats is highly remarkable and the warble flies must be considered as one of the serious causes of losses in the region. According to the seasonal pattern of the life cycle of the parasite, we suggest that early-autumn and late-summer are the best occasions for prophylaxis with systemic insecticides in the cattle and goats, respectively. Since the peak of the spinal canal infestation in cattle is coincident with mid-winter, administration of insecticides at this time could be followed by complications such as damage to the central nervous system.