Abstract: Parascaris equorum (ascarid; roundworm) is a common nematode parasite which occurs in the small intestine of immature horses world-wide. Adult female ascarids lay eggs in the small intestine, and these eggs pass into the environment within the feces of the host. P. equorum is one of the rare nematodes which induce absolute acquired immunity. Most horses become immune during the first year of life, so patent ascarid infections are rarely diagnosed in horses over two years of age. The aim of this study was to determine the prevalence of infection with parascaris equorum in Tehran’s riding clubs. The prevalence and rate of infection was determined based on the coproscopic examination. Fecal samples were tested for the presence of parascaris using suspend method. In this investigation, 442 fecal samples of horses from North-East of Tehran’s riding clubs examined. From the viewpoint of parascaris roundworms, fecal samples were obtained from each box separately and send in containers with plastic lid. The samples were then transferred to the parasitology lab for further examination. The infection was recognized based on the observation of parascaris eggs in coproscopic examination. The infection rate in the foal in this study was zero percent. Another interesting result was increasing the infection rate in horses of 10 years or even older and gelding. Out of 442 samples, the infection rate in the samples taken in summer, was 3.16%, and in the fall, winter and spring it was 2.4%, 10% and 3.16% respectively. The infection rates in relation to the age, sex, excursion condition, seasons and deworming programs was studied too. The results showed that the local and the imported horses should be monitored parasitologically, because endoparasites may create a major epizootiolocall problem when these animals are kept in an organic raising system. Deworming program is to be continued with proper methodology, dose and throughout the productive age of the horses.