Some species of late Paleozoic bellerophontacean gastropods attained unusually large sizes. Yochelson (1960) described several Permian species from the American Southwest having shells more than 50 mm long, and noted that the largest North American specimen is an incomplete Leonardian steinkern more than 90 mm long, from the Glass Mountains of Texas. A Permian species of Bellerophon from Japan is up to 120 mm wide (Yochelson, 1960), and several Mississippian and Pennsylvanian species in the 90+ mm range are also known (Weir, 1931; Kues, 1987). Steinkerns of this size are not uncommon in Wolfcampian and Leonardian formations in New Mexico, but because of lack of preserved shell material and/or occurrence in massive limestones they are rarely collected and little is known about them. In this note we report and illustrate an unusually well preserved but incomplete Guadalupian bellerophontid from southern New Mexico. It is considerably larger than the specimens mentioned above, and appears to be the largest late Paleozoic bellerophontid yet reported.