Biological and molecular (sequence analyses of COI and D2 region of 28S rDNA) evidence indicates that Phytoptus avellanae Nalepa (Acari: Eriophyoidea: Phytoptidae) represents a complex of two cryptic species associated with Corylus avellana L. (Betulaceae). To date, the two species have not been reliably separated on the basis of discrete morphological characters, except for slight differences in body size, shape and color of the adult females, and the occurrence of an atypical Tegonotus-like nymph. One species has a vermiform nymph, is gall-making and causes abnormal bud swelling; the other species, characterized by a Tegonotus-like nymph, is vagrant on leaves and in catkins, shelters beneath bud scales, and damages flowers and fruits. The present study was aimed at identifying morphological characters that would allow discrimination between the two taxa. Populations of mites were collected from typical bud galls, as well as from leaves and water traps in three different Italian areas (Apulia, Sicily and Latium). Specimens were mounted on microscope slides and studied under phase contrast microscopy (Olympus, BX50) mostly under 40x Plan 0.65 Ph2 and 100x UPlanFl 1.30 oil Ph3 objectives and photographed using a Canon Mark III digital camera connected to a computer; the images were processed using Adobe Photoshop 2020 (rel. 21.2.2, 20200807). Examination of pharate adults as well as adults eclosing from the exuviae of the atypical nymphs revealed the presence of 7-8 smooth dorsal semiannuli in the posterior opisthosoma. This feature is also documented in figs 3E and F by Cvrkovic et al. (2016), although it was not discussed on. In contrast, specimens collected from bud galls exhibited dorsal semiannuli entirely microtuberculate or, at most, with three smooth dorsal semiannuli in the posterior part of the opisthosoma. No neat additional morphometric characters among 75 measured allowed reliable separation of the two species, except for the morphology of the epigynal coverflap base. In the gall-making species, the coverflap is entirely smooth, whereas in the vagrant species it bears a restricted ornamented area consisting of 8–12 very short dashes. These markings may be mistaken for internal genital apodemes and can be difficult to discern when the coverflap is folded.