Although a number of excellent papers have appeared on the brains of teleosts in general, very few studies have been reported on the brains of deep sea fish. A paper by Trojan (1906) gives but a superficial survey, while that by Gierse (1904) deals primarily with the external form of the brain, the accessory organs of the brain, and the peripheral cranial nerves. The purpose of this investigation is to present a somewhat detailed description of the cell areas and fibre pathways, especially of the diencephalon and mesencephalon, the object being to determine what modifications have taken place in connection with their unusual environment. Very little is known about the natural history of these fish. In a personal communication concerning them, Professor Burr says, “The Diaphus dumerili were collected by the Bingham Oceanographic Expedition off the Cat Islands, the Bahamas, in 1927. Of these there are six specimens, Bingham Collection Nos. 9, 21, 36, 37, 38, and 39. The two Saurida suspicio , Bingham Collection Nos. 19 and 40, were collected also off Gat Island, the Bahamas. The Bathypterois articolar phenox is a very rare specimen; it was collected at N. 20°— 16' W. 71°— 18', at a depth of from 900—945 fathoms, also in 1927.”