Although some ecological theory predicts that herbivore density should increase with plant quality or biomass, few studies have directly measured the response of multiple groups of grassland herbivores to changes in plant production. In this study, I experimentally manipulated plant biomass by fertilizer addition and measured density and diversity of small mammals and insects (primarily Rodentia and Orthoptera). A total of 245 small mammals representing 9 species were captured during the study. For the three most common species, 82% of mammals comprised of Peromyscus leucopus and Peromyscus maniculatus while Sigmodon hispidus dominated captures of mammals > 100g. The density of the rodent community was higher in fertilized than in unfertilized plots. However, I detected no effect of increased plant biomass on small mammal diversity. The body mass of S. hispidus and P. leucopus was higher in the fertilized than in unfertilized plots but no effect was observed for the P. leucopus. For insect herbivores, grasshopper biomass, density as well as diversity were higher in fertilized than unfertilized plots. Collectively these results from multiple herbivore groups are consistent suggesting that plant productivity influences the composition of small mammal and grasshopper assemblages in grassland systems.