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Herbivory by medium-sized rodents is of great importance in Monte Desert communities. In this paper we study changes in the plant cover, plant composition and diversity in two creosotebush communities of the Monte Desert induced by the herbivorous subterranean rodent Ctenomys sp. (tuco-tuco). We studied two sites: a community where only Larrea divaricata is dominant (site 1), and a mixed community dominated by L. cuneifolia and L. divaricata (site 2). On both sites we studied two types of areas: highly disturbed by Ctenomys, and less disturbed by this species. In the highly disturbed areas, herb cover decreased in both plant communities (6.4 vs. 14.1%, and 15.1 vs. 32.4%, respectively), whereas shrub cover diminished significantly only in site 1. The foraging activity of this herbivore affected the cover of Pappophorum caespitosum, Aristida sp. and Digitaria sp, Herb diversity was significantly different between highly and less disturbed areas in both communities considered, although it was higher in the less disturbed area on site 1, and in the highly disturbed area on site 2. Shrub diversity did not vary significantly between highly and less disturbed areas. (C) 2001 Academic Press.