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E. Iehmanniana Nees was seeded in Nacunan Reserve in 1976. The linear rate of spread, the abundance of this exotic grass and the impact of its introduction on the native perennial grass community were evaluated after 24 years seeding. Sites dominated by the exotic grass and those including only native vegetation were compared. The Mann-Whitney test was applied to compare grass canopy cover, number of species per plot and abundance of the exotic grass between the mentioned sites. The exotic grass spread 32 m year(-1). It was present in tracks where the native woody vegetation had been removed, and in 45 % of the sites located in undisturbed areas adjacent to tracks. The total native grass canopy cover was significantly reduced in the sites where lovegrass was present. The number of total grass species per plot was not affected by the presence of the exotic grass. Pappophprum caespitosum, the dominant grass in Nacunan, and Sporobolus cryptandrus presented lower canopy cover in the sites dominated by the exotic species than that in the undisturbed ones. The introduction of E. Iehmanniana has had a negative impact on the composition of the native perennial grass community in the disturbed sites of the Reserve.