Background:& nbsp;This study was conducted to determine the prevalence and intensity of nematode infections in sheep located in northeastern Iran from Apr 2018 to Mar 2019. Methods:& nbsp;Gastrointestinal nematodes of 300 sheep were inspected. The season of slaughter, anatomic location where the parasite was located, the animal & rsquo;s sex, infection prevalence and intensity were recorded. Seasonal differences in arrested larvae numbers also were assessed using Cochran 's Q test. Results:& nbsp;Overall, 4,331 adult nematode specimens were collected. Among the examined sheep, 53% (159/300) were infected with one or more nematode species. Among infected sheep, 42.8% were infected with a single species of nematode, 26.4% were infected with two species of nematodes, and 30.8% were infected with three or more species of nematodes.& nbsp;Marshallagia marshalli& nbsp;(13.3%) was the most common nematode recovered from the abomasums of infected sheep, while & nbsp;Trichostrongylus vitrinus& nbsp;(4.6%) was commonly recovered from the small intestines, and & nbsp;Trichuris ovis& nbsp;(25.6%) was commonly recovered from the large intestines. In total, 463 arrested larvae were found in the abomasums of 7.5% of infected sheep and 104 arrested larvae were found in the small intestines of 8.8% of infected sheep. A significantly higher numbers of arrested larvae were found in summer compared to autumn (P< 0.001). Conclusion:& nbsp;Intestinal parasites continue to be a problem for sheep in northeastern Iran and additional control measures need to be explored.
