These data were obtained as part of a field study on social behavior of the prairie vole, Microtus ochrogaster in alfalfa. The study was conducted in two adjacent 1-ha alfalfa (Medicago sativa) fields within the University of Illinois Biological Research Area (Phillips Tract). Social groups were monitored over a 63-month period, from March 1982-July 1984 in field 1 and October 1983-May 1987 in field 2, by locating underground and surface nests and subsequent trapping twice weekly. 4-5 live traps were set around the entrances to underground nests of social groups. Each month, the study sites were also trapped at a 10-m grid interval as a part of an ongoing 25-year trapping study (Getz, 2024).