The structure of small mammal communities in nine North American grassland types is examined quantitatively utilizing data collected during a 4-year period. Small mammals in each grassland type are divided into groups according to species, size, reproductive strategy, life form, seasonality of activity, and diet. Observed spatio-temporal patterns of faunal composition are related to differences in structure of the different grassland ecosystems. The structural differences identified as being most important are primarily those related to latitudinal and moisture gradients. Temporal variability of small mammal community composition is characteristic of all grassland types. The degree of temporal variability is not easily related to structural differences of the grasslands studied.