The Casa Grande ruin, situated near Gila river, in southern Arizona, is perhaps the best known specimen of aboriginal architecture in the United States, and no treatise on American antiquities is complete without a more or less extended description of it. Its literature, which extends over two centuries, is voluminous, but of little value to the practical scientific worker, since hardly two descriptions can be found which agree. The variations in size of the ruin given by various authors is astonishing, ranging from 1,500 square feet to nearly 5 acres or about 200,000 square feet in area. These extreme variations are doubtless due to difference of judgment as to what portion of the area covered by remains of walls should be assigned to the Casa Grande proper, for this structure is but a portion of a large group of ruins.So far as known to the writer no accurate plan of the Casa Grande ruin proper has hitherto been made, although plans have been published; and very few data concerning the group of which it forms a part are available. It would seem, therefore, that a brief report presenting accurate plans and careful descriptions may be of value, even though no pretention to exhaustive treatment is made.