In preparing the part dealing with the Diprionidae in a synopsis of the British sawflies I came upon several morphological characters that, if used or made more use of, would greatly facilitate the naming and classification of this economically important group of insects. A comparison between Neodiprion and Diprion (s.l.) revealed at once striking differences in the shape of the meso-scutellum, the sculpture of the abdomen and the anal cell in the hindwing, characters easily visible in the field and therefore of great importance to the economic entomologist. Diprion pini, L., and similis, Hartig, were found to differ from the other Diprion species, among other things, in their small cenchri, a character easily visible in the field, and the genus has been divided.