We examined long-term (1978–1999) population fluctuations and their influence on community structure in 6 species of rodents that are syntopic in old-field habitat in northeastern Kansas. Populations of all species fluctuated severalfold in abundance, with Sigmodon hispidus, Peromyscus maniculatus, and Reithrodontomys megalotis exhibiting annual cycles. Multiannual periodicities were evident for Microtus ochrogaster, P. maniculatus (3.5–4 years), and P. leucopus (7 years). Only Synaptomys cooperi lacked a discernable pattern in its abundance. Monthly abundances of 6 of 15 species-pairs positively covaried, suggesting similar responses to long-term environmental variation. In addition, several species-pairs showed time-lagged cross-correlations that indicate regular annual cycles, which were out of phase. Because of this variation in population dynamics within and among community members, the structure of this community varied considerably over time, indicating a nonequilibrial community. However, when looking at the community as a whole, 2 reoccurring long-term patterns in community structure emerge: an annual and a 3.5-year cycle. Annual cycles in community structure occurred because 3 of the 6 species showed annual cycles, whereas the 3.5-year cycle in community structure was probably due to the overwhelming influence of fluctuations in abundance of the numerically dominant prairie vole.