If aggressive behavior regulates population size in small rodents (as claimed by Chitty 1967), periodic fluctuations must be associated with behavioral changes. In an attempt to test this hypothesis, exploratory activity and aggressive behavior were measured in 645 male Microtus ochrogaster and 570 M. pennsylvanicus of 7 populations in southern Indiana from August 1965 to September 1967. Exploratory activity was measured in an open—field test under daylight conditions, and aggressive behavior was measured by fighting voles in pairs in a neutral fighting arena. Exploratory activity could not be related to the individual attributes of home range size or duration of life in males of either Microtus species. On the population level, mean exploratory activity could predict the mean rate of population growth in M. ochrogaster populations, but not in M. pennsylvanicus populations. Aggressive behavior changed significantly over the population cycle in both species, and males from peak populations were most aggressive. Multiple discriminant analysis was used to characterize the behavior patterns of males from increasing, peak, and declining populations. These discriminant scores predicted the mean rate of population growth in M. pennsylvanicus populations but not in M. ochrogaster populations. Aggressive behavior types characteristic of increase, peak, and decline phases showed differential survival rates, but no association was found with body weight or transferrin genotype in either species. Home range size was related to aggressive behavior type in M. ochrogaster but not in M. pennsylvanicus. These data on two species of Microtus do not refute the hypothesis that aggressive behavior may act to regulate population size. However, there is no information on what causes these behavioral changes, and the next step is to see if these changes occur in other populations of these two species and to determine whether they are genotypic or phenotypic.