Effects of 74 episodes of extreme weather on stoppage of population growth and resulting amplitudes of annual population fluctuation of 39 fluctuations of Microtus ochrogaster and 20 fluctuations of M. pennsylvanicus were studied over a 25 y period in east-central Illinois. Episodes of extreme weather may have stopped population growth of only six M. ochrogaster fluctuations and of two M. pennsylvanicus fluctuations. Cessation of growth of only one population fluctuation (M. pennsylvanicus) could be attributable solely to an episode of extreme weather. Episodes of extreme weather occurred during 62% of the increase phases of M. ochrogaster population fluctuations and 75% of those of M. pennsylvanicus, with no associated cessation of population growth. We conclude that episodes of extreme weather were not a primary factor responsible for cessation of population growth or variation in amplitudes of population fluctuations of either M. ochrogaster or M. pennsylvanicus.