The Australian Nothomyrmecia macrops is the most primitive living ant. Until recently rediscovered, it was known only from two 46-year-old specimens, both workers. All developmental stages and adult castes are now known. Adults have stridulatory organs placed ventrally, between abdominal sternites III (with plectrum) and IV (with stridulitrum), differing from all other Hymenoptera, where these organs are dorsal. The chromosome number is the highest recorded for Hymenoptera (2n = 92). Virgin queens are brachypterous. Other anatomical and behavioral features are reviewed. Most are primitive for ants, many being shared with Myrmecia, another primitive Australian genus. Fundamental differences in abdominal structure place Myrmecia near the base of the poneroid phylad of ant subfamilies, while Nothomyrmecia is a primitive formicoid.