We sampled the small mammal fauna of 18 successional old-fields, aged 2–57 years since abandonment from agriculture, and examined patterns of diversity and abundance along successional and productivity gradients. Through time, vegetational standing crop, cover, litter, and plant species composition changed, but vegetational physiognomy did not; thus, we examined small mammal populations within a grassland chronosequence. Small mammal densities were low and were not strongly associated with successional time. Species richness and density, however, were strongly associated with vegetational standing crop and nitrogen content. Peromyscus leucopus densities were best correlated with variability in vegetation (SD vegetational biomass), whereas density of Microtus pennsylvanicus and Sorex cinereus were best correlated with vegetational nitrogen content. Overall, the data suggest that the diversity and abundance of small mammals were limited by nitrogen.