1. Small mammal population fluctuations, cyclic or not, have been an ecological puzzle and a source of heated debate among ecologists. Identifying the demographic parameters that covary closely with density changes can help elucidate the underlying causes of population fluctuations, but few studies have reported rigorous estimates of these parameters. 2. We applied capture-mark-recapture analysis to twice-weekly trapping data from a long-term study of a fluctuating Microtus ochrogaster (prairie vole) population in Illinois, USA to estimate stage-specific apparent survival and maturation rates. We also estimated population density, fecundity rate, age at maturity and life span on a weekly basis. 3. Survival, maturation and fecundity rates exhibited phase-related changes during major density fluctuations, but they showed density-independent temporal variations during the prolonged low-density phases. Among these variables, maturation and juvenile survival rates covaried most closely with population density. 4. These results suggest that phase-related changes in maturation and juvenile survival rates are likely to be the main demographic factors driving the dynamics of our study population. 5. Phase-related changes in maturation rates provide a plausible demographic explanation of density fluctuations in our study population. Our results suggest that direct predation may not be necessary for large-scale fluctuations in M. ochrogaster abundance.