This series contains biographical information on Elrod and members of his family that was produced by Elrod, his daughter Mary, and numerous other individuals. Elrod appears to have made a comprehensive genealogical study of his ancestors, and his notes comprise much of the series. There is also a file with printed material including graduation and wedding announcements and clippings of Elrod's obituary.

This series contains mix of personal and professional correspondence that ranges from the mundane to the highly informative. Letters include a great deal of University of Montana business, as well as letters to and from organizations that reflect Elrod's wide and varied interests and involvements. Organizations and persons represented include the United States National Museum, the Forest Service, George Bird Grinnell, and W.A. Clark and other U.S. senators. There is also a great deal of family correspondence, including the letters he wrote to and received from his wife and daughter during his frequent travels. There are carbon copies of many of his outgoing letters after 1910.

This series contains Elrod's correspondence with and publications about the American Association of University Professors, the American Society of Zoologists, the American Microscopal Society, and the Entomological Society of America. There are also files concerning buffalo, particularly the National Bison Range, and on Missoula organizations like the University of Montana's Author's Club and the Cosmos Club. There are numerous articles he wrote to describe and to promote the natural beauty of Montana. The files are a mix of Elrod's own work and work that he collected from other authors, including many of his students. Notable among those are papers by Jeannette Rankin and Harold Urey. The series also contains the manuscripts of Elrod's books on butterflies and birds of Montana. Materials are subdivided into three categories: General, Montana, and Natural History. Within those categories, materials are arranged alphabetically by subject. Names of other authors are noted as available.

This series consists mostly of materials produced by Elrod during his long involvement with the development and promotion of Glacier National Park, most notably as park naturalist. The materials cover the pre-beginnings and early days of the park and offer documentation of its rise in popularity. It contains extensive research notes, drafts and final copies of publications, and a substantial body of correspondence with persons interested in information on the Park. Included are the manuscripts of books he wrote, including the first guide to the Park, a book on wildflowers, and a book on Grinnell Glacier. There are also notes for a speech he gave on George Bird Grinnell. There are a few materials written by other authors, mostly on natural history subjects.

This series focuses on Elrod's very central involvement with the early history of the University of Montana. There is extensive documentation of general university events he was involved with as a faculty member; materials specific to course offerings and student work in the Biology Department; and numerous materials relating to the development and offerings of the University of Montana Biological Station.

Elrod's materials from the Biology Department consist mostly of class and lecture notes for courses in bacteriology, biology, ecology, entomology, eugenics, heredity and evolution, histology, hygiene, ontogeny, physiology, and zoology. Materials on general campus events demonstrate the depth and breadth of Elrod's involvement with the campus. Included are files on the establishment of Aber Day, the issue of statewide university consolidation in Montana in the 1910s, the Arthur Fisher case, the establishment of the university museum and the , and athletics, including the annual Interscholastic meets. Materials are arranged alphabetically within University and University of Montana Biological Station categories.

This series contains newspaper clippings collected by Elrod on the University of Montana, the opening of the Flathead Valley to settlement in 1909, and on Elrod. The latter category includes articles on the dedication of the Elrod Bird Sanctuary.

This series contains a mix of personal and University-related financial records. It includes checks and bank statements and records relating to sales of Elrod's picture post cards. University-related materials include records of the University lecture series and seed sales from the Biology department. There is also an account book relating to the sales of Dean Stone's book, .

This series contains an eclectic group of Elrod's writings. Most of his works on natural history are elsewhere, but a few are here, including short articles on mountaineering and camping for girls. There are a few folders of his poetry, and a number of field notes on specimen collections, the Biological Station, and Glacier National Park. In addition, there is a typescript of the Harriet Saunders diary (original dated 1863), field notes taken around Bloomington, Illinois, and lecture notes from the University of Montana.

This is one of five series that contain photographs from Elrod's considerable collection. The photographs depict the Elrod family, Missoula, the University of Montana, the Swan Valley, the Mission Valley, the Flathead Valley, Crow and Salish-Kootenai tribal members, flora and fauna of western Montana, and other regions of the American west in the early 1900s. Significant subjects include Glacier National Park and the University of Montana's Biological Stations at Big Fork and Yellow Bay. A large portion of the Yellow Bay photographs were taken during the construction and early years of the station. Elrod also took many photographs of boats that ferried people and supplies to the Biological Stations from Flathead Lake, including the first motorboat in Montana, .

Images from several other photographers are represented in the series. There are photographs Elrod collected from his contemporaries, including R.H. McKay, Ralph E. DeCamp, Kenneth D. Swan, Frank M. Ingalls, T.J. Hileman, R.E. Marble, and others. There are also photographs from Elrod's students. Oversize boxes contain 11"x14" prints, photograph albums, and photograph collages from Yellow Bay. Many of these images duplicate classfied photographs in Series X. A notable oversize image features the first x-ray in Missoula, Montana