Aceria tristriatus (Nalepa, 1890) is one of the major pests of walnut (Juglans regia L.: Juglandaceae) in Iran, especially in western of country. It causes galls on leaves that mainly concentrated along the main and lateral veins. The gall forms toward the upper surface of the leaf with the opening on the lower side. This paper reports the re– description of adult stages (protogyne, deutogyne and male) and description of immature stages of walnut leaf gall mite. Furthermore difference between all mobile stages in this species based on morphological characteristics is provided. Mites were collected from walnut trees infested with this pest in Hamedan and Lorestan provinces, western Iran
