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1. Species-rich grasslands are of high conservation value because of the diverse floral and faunal assemblages they support. Intensive agriculture has resulted in the large-scale loss and fragmentation of this habitat throughout the UK and Europe.2. Diversification of species-poor grasslands has proved to be problematical due to seed limitation, lack of microsites for germination and competition from established species.3. Studies of semi-natural grasslands in Britain and Europe suggest that the presence of hemiparasitic angiosperms of the genusRhinanthusis correlated with local decreases in productivity and an associated increase in species richness.4. In 1998 a randomized block experiment was established to examine: (i) the practicality of introducingRhinanthus minorinto a productive grassland from seed; (ii) the rate of establishment and colonization of the introduced populations; and (iii) the relationship between the frequency of the parasite and the establishment and persistence of introduced forb species.5. Establishment ofRhinanthuswas less likely at the lowest sowing rate (0·1 kg ha−1) and population growth was initially slow compared with the higher seed rates (0·5 and 2·5 kg ha−1). After the first year, populations grew rapidly and colonized the unsown plots, so that after 4 years there was little difference inRhinanthusfrequency between treatments.6. In October 2000, 2 years after sowingRhinanthus, a native wildflower seed mixture was oversown into a number of the treatments at 5 kg ha−1. The mixture comprised 10 forbs of which nine species were previously not present in the grassland. All forbs established with up to seven species m−2. This resulted in an increase in mean richness of sown species of 0·8 to 2·2 m−2 and in a cumulative richness per plot of 1·4 to 5·8 m−2.7. In both 2001 and 2002 there was a strong, inverse relationship between the frequency ofRhinanthusand sward height. In 2002, there was evidence of a threshold frequency of the hemiparasite above which there was a significant reduction in grassland productivity.8. The frequency ofRhinanthusin 2001 had a significant beneficial effect on the persistence of seven of the 10 sown forbs in the following year. This probably reflects a reduction in competition from the established grassland species at the vulnerable seedling stage, together with death of the hemiparasite leading to a higher number of gaps for colonization.9. Synthesis and applications.The introduction ofR. minoris a practical management tool to facilitate the colonization of species-poor, productive grasslands by desirable species. This is achieved by the hemiparasite modifying the competitive relationships between the component species, and increasing establishment and survival of introduced species. This species has a potentially central role in determining the structure and dynamics of grassland communities.Journal of Applied Ecology(2004)41, 880 –887 [ABSTRACT FROM AUTHOR]