Bird-atlas data were used in conjunction with information on the extent of commercial afforestation with alien trees in Mpumalanga Province, South Africa, to assess the effect of afforestation on the avifauna of this region. Ninety species of birds characteristic of grassland habitats occur in the province. Twenty-five of these species are of conservation concern and 10 are globally threatened. A separate suite of 65 species associated with woodlands or forests occurs in the province and benefits from afforestation or at least the spread of alien trees. The areas of highest species diversity of grassland birds overlap extensively with the areas of greatest afforestation and potential additional afforestation. The species diversity of grassland birds generally, and globally threatened grassland birds in particular, is significantly and negatively correlated with the extent of afforestation. Afforestation apparently has a negative impact on grassland bird diversity even when the percentage area under plantation is relatively small. A comparison of the avifaunas negatively and positively impacted by afforestation reveals that the grassland community has more species and is richer in both endemics and threatened species than the plantation community. Any further afforestation in Mpumalanga Province is likely to contribute substantially to the potential extinction of many bird species there, including several globally threatened species. Keywords: birds, forestry, grasslands, South Africa.