Gasterophilus spp. (Diptera, Oestridae) are known as botflies and their larvae cause gastrointestinal myiasis in equids. In February 2015, nine mares from a free-range herd of 80 Turkmen horses in northeastern Iran became acutely ill and died. To determine the cause of death all of them were necropsied. In gross examination of gastrointestinal tract, numerous botfly larvae attached to mucosal surface of esophagus and non-glandular part of the stomach were observed. All animals necropsied were infested with these larvae. Parasitological examination showed the larvae were third stage larvae of Gasterophilus pecorum. This is the first record of this botfly species in horses from Iran. Although these larvae were not determined as the causative agents of acute illness and death, but with regard to their adverse health effects, treatment of horses and donkeys with macrocylic lactones was recommended in infested areas.

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