Species composition and density were studied in two areas on a watershed in the Chihuahuan Desert in southern New Mexico. The upper portion of the watershed, a creosotebush community, supported "resident" populations of Dipodomys merriami, Perognathus penicillatus, Neotoma sp., and Onychomys torridus. The lower portion of the watershed, which drained into a grass-covered dry lake, supported Sigmodon hispidus, Reithrodonomys megalotis, Dipodomys ordii, and Spermophilus spilosoma, in addition to the species found in the creosotebush area.... Changes is species diversity in tie was correlated with rainfall and productivity. The dynamics of these Chihuahuan Desert rodent communities are considered in terms of immigration and extinction as the suitability of habitat islands change through time.