SUMMARY Previously, Cox (1978) synonymized Rhizoecus perprocerus De Lotto, 1961 with Mo- rrisonella graminis Hambleton, 1946 (now known as Rhizoecus graminis), based on a morphological comparative study of the adult female. Later, Kozar & Konczne Benedicty (2007) considered the synonymy by Cox (1978) as an incorrect synonymy. In the present study, R. perprocerus is once more synonymized with R. graminis after reexamination of the morphology of the type material of R. graminis and R. perprocerus. The taxonomic value of the shape and length of the genitalia, the distribution of ventral tritubular ducts on the abdomen, and the size of the clavate sensory setae in the antennae are discussed.