A new species of the genus Deladenus, D. gilanica n. sp. collected from Siahkal forests of Northern Iran, is described and illustrated. The new species is characterized by its 314 to 422-µm-long body, eight incisures in the lateral field, 7.5 to 8.0-μm-long stylet, position of the excretory pore, at the level of the hemizonid, both posterior to the nerve ring 61 to 76 µm from the anterior end, and conical tail with pointed tip. Based on morphology and morphometrics, the new species can be compared with eight known species of the genus: D. aridus, D. durus, D. obesus, D. oryzae, D. processus, D. wilsoni, D. proximus, and D. posteroporus. Evolutionary relationships of the new species with other Deladenus species were assessed with sequences of the D2 to D3 expansion regions of 28S rRNA and partial 18S rRNA gene. The phylogenetic analysis showed that D. gilanica n. sp. is genetically distinct from other included species. Deladenus gilanica n. sp. is a member of the D. siricidicola species complex and close to D. canii, D. nitobei, and D. siricidicola. Typologically, these species share the relative position of the excretory pore to the hemizonid.
