Natural prairie has become uncommon in North America, making the revegetation of disturbed areas by native species a desirable goal. Alien species are often introduced for revegetation because of their abilities to stabilize and nitrify soil. The objective of this study was to test whether these attributes of introduced species would allow them to promote the recovery of native vegetation. Seven treatments (six commercially-available mixtures of introduced species and an unseeded control) were applied to a randomized field experiment in disturbed mixed-grass prairie in south-west Manitoba, Canada. Sampling eight years later revealed that introduced species suppressed native vegetation. Introduced species did not aid revegetation: plots seeded with introduced species did not produce significantly higher standing crop or below-ground biomass than did unseeded plots. Unseeded plots had the lowest frequency of bare ground. Allowing prairie to revegetate without sowing introduced species produced both the highest cover of bare ground and the greatest abundance of native species.