Factors influencing initiation of population fluctuations of Microtus pennsylvanicus were studied in alfalfa and bluegrass habitats for 25 years. Increased survival during spring and summer appeared to be the most important factor associated with initiation of a population fluctuation. The proportion of reproductively active adult females did not influence initiation of population fluctuations. The interval between fluctuations was not correlated with density of the previous population fluctuation. We propose that population fluctuations were initiated by the net effects of relaxation of predation pressure of multiple generalist predators, which occurred erratically across years.