Ancistrocerus capra was described by de Saussure in 1857 from North America and then was synonymized with the Palaearctic A. antilope (Panzer, 1798) by Bequaert in 1944. Although these species share a combination of two characters (impunctate and shining metapleuron and lateral surface of the propodeum and a bifurcate apex of the aedeagus) not known in other species of Ancistrocerus, they are clearly different in the structure of the clypeus (especially in the male) and the male genitalia (structure of the volsella, general shape of the aedeagus, and the structure of its ventral lobe). Thus, A. capra is again recognized as a distinct species in the present contribution. This species is distributed in the United States and Canada while A. antilope has a trans-Palaearctic distribution. Two subspecies of A. antilope known from North America are synonymized with A. capra: A. antilope navajo Bequaert, 1925 (new synonymy), and A. antilope allegrus Bequaert, 1944 (new synonymy); the taxonomic status of A. capra spenceri Bequaert, 1944, remains unclear. Bionomics of A. antilope and A. capra are similar; particularly, these species share an association with apparently the same species of symbiotic mites, Kennethiella trisetosa (Cooreman, 1942) (Sarcoptiformes: Winterschmidtiidae), and an unusual mating behavior correlated with this association (first of all, several copulations per pair). A difference, however, exists in the number of generations per year and the sex ratio: A. antilope has a single (overwintering) generation with a female-biased sex ratio while A. capra has an overwintering generation with a male-biased sex ratio and a summer one with a female-biased sex ratio.