Vole population dynamics are well characterized in Europe, Fennoscandia, and Japan, but long-term studies of vole dynamics in North America are rare and much needed. Summer relative abundance of southern red-backed voles (Clethrionomys gapperi) fluctuated periodically during 22 years (1983–2004) at the Holt Research Forest, Maine, suggesting an outbreak or cyclic dynamic. The time series had a 4.4-year period, and 2nd-order lagged relative abundance was significant in an autoregressive model. This was consistent with vole series in Fennoscandia and Japan. Above-average white pine (Pinus strobus) seed crops preceded C. gapperi peaks in 4 of 5 peak phases. In a model including P. strobus seeds, both seeds and 2nd-order lagged abundance terms were significant. Relative importance of P. strobus seeds to this omnivore needs to be established. High and low phases of C. gapperi corresponded with those of sympatric white-footed mice (Peromyscus leucopus). The 2nd-order autoregressive process, 4.4-year periodicity, and synchrony with P. leucopus indicated the C. gapperi dynamic was cyclic. This was a unique finding for the northeastern United States.