Systematics and distribution of Brachistosternus alienus Lonnberg (Scorpiones Bothriuridae). New data are provided on the distribution and systematics of Brachistosternus alienus Lonnberg, a species that has been misinterpreted in the past. It occurs in the northern part of the Patagonia phytogeographic province and in the southern part of the Monte phytogeographic province. It can be differenciated from the two sympatric species of Brachistosternus by its smaller size and its lower number of pectinal teeth. Brachistosternus alienus belongs to the subgenus Leptosternus Maury, according to the current classification of the genus.