From 1996 to 2004, we studied population dynamics of a rodent assemblage in semiarid habitats in Lassen County, California. Abundances of rodents appeared affected by fluctuations in precipitation from a high in 1996 to average and slightly below-average levels in subsequent years; perhaps reflecting this, assemblage composition also changed dramatically during the study period. California kangaroo rats (Dipodomys californicus) declined from abundant to extremely rare, abundances of deer mice (Peromyscus maniculatus) fluctuated greatly during the study period, and, perhaps most notably, populations of dusky-footed woodrats (Neotoma fuscipes) appeared to decline to local extinction, possibly allowing for the subsequent colonization of desert woodrats (N. lepida). These changes appeared to represent natural variation in numbers and composition in an ecologically dynamic ecotonal region in response to multiyear changes in precipitation.