The possible impact on Microtus pennsylvanicus of ground applications of Orthene^® insecticide was investigated in old-field habitats in northern Maryland during 1982 and 1983. The treatment grids in 1982 and 1983 were sprayed at 0.62 and 0.82 kg active ingredient/ha, respectively. A capture–recapture design robust to unequal capture probabilities was utilized to estimate population size, survival, and recruitment. Data on reproductive activity and relative weight change were also collected to investigate the effect of the insecticide treatment. There were no significant differences in population size or recruitment between control and treatment grids which could be directly related to the treatment. Survival rate was significantly lower on the treatment grid than on the control grid after spraying in 1983; however, survival rate was higher on the treatment grid after spraying in 1982. Significantly fewer pregnant adults were found on the treatment grid after spraying in 1982, whereas the proportions of voles lactating or with perforate vaginas or open pubic symphyses were slightly higher or remained unchanged during this period. Relative weight change was not affected by the treatment. Results do not indicate any pattern of inhibitory effects from the insecticide treatment. Field application of Orthene^® did not have an adverse effect on this Microtus population.