In the framework of the BIOTA‐Southern Africa Project Arge deckerti n. sp. is described. It is compared with the very similar species Arge stuhlmanni (Kohl 1893). The morphological characteristics of these species are illustrated, and their distribution in southern and East Africa is given. A record for Athalia ustipennis Mocsáry in Namibia is presented. Keywords: Namibia, South Africa, southern Africa, East Africa, sawflies, Hymenoptera, Symphyta, Argidae, Arge, Athalia.