The objective of this study was to determine the prevalence of metacercarial infection in plathyfish imported to Iran. A total of 100 imported live common platyfish (Xiphophorus maculatus) from approximately 24 shipments were collected from a quarantine facility in Iran in 2007 and examined in our veterinary laboratory. Among 800 examined gill arches about 1300 cysts were found in different locations of gill filaments and were identified as a heterophyid parasite metacercaria Ascocotyle (Ascocotyle) tenuicollis Price, 1935. The prevalence of this parasite was 58%, the maximum and minimum quantity of metacercarial cysts per infected platyfish were 70 and 10, respectively. The adult worms are non-pathogenic and are intestinal parasites of piscivorous birds (e.g. Heron and Egret) and mammals. The first intermediate hosts are snails and the second intermediate hosts are centrarchiid, cyprinodontid, mugilid, and poeciliid fishes. Histopathologic examination of the infected fish gills revealed cartilage proliferation around metacercarial cysts, hyperplasia, hypertrophy and fusion in the affected gill filaments. To our knowledge, this is the first report of infection with Ascocotyle tenuicollis in the imported platyfish in Iran.