in sub-humid grasslands moderate grazing by large herbivores may promote increases in plant diversity. our objective was to evaluate changes in floristic diversity and floristic composition at different grazing intensities by large herbivores in mountain grasslands of the ventania system, buenos aires, argentina. sampling comprised an area of 200 ha, on needlegrass grasslands grazed by feral horses and cattle. nine 1 ha plots were selected to represent a grazing intensity gradient, and we evaluated plant species cover in order to describe the plant diversity pattern, species richness and cover, and floristic composition in relation to the grazing intensity gradient. plant diversity was greater at moderate grazing intensity mainly due to the increment of dicots and cool-season grasses richness. in terms of cover, grazing only increased dicots' aerial cover. grazing was associated with changes in the floristic composition of needlegrass grasslands, through inducing the replacement of cool-season grasses (e.g. piptochaetium hackelii, briza subaristata, nassella filiculmis) by an unpalatable warm-season grass (aristida spegazzinii). in mountain grasslands of the ventania system, moderate grazing intensity appears to favor plant diversity by increasing dicots and cool-season grasses richness, whereas high grazing intensity may promote the replacement of palatable cool-season grasses by unpalatable warm season grasses.