This collection consists two subgroups, Research Materials and James Fergus Family. The Research Materials subgroups consists of both primary and secondary research materials on the history of central Montana, including Oscar Mueller's correspondence, writings, research notes, and subject files on the Judith Basin. Included also are writings by Mueller's son George D. Mueller, his wife Josephine Cook Mueller, and by W.E. "Limestone" Wilson. The James Fergus Family subgroup includes James Fergus family materials collected by Oscar Mueller during his research.