Alien plants are rapidly invading natural areas in South Africa but their impacts on biodiversity, particularly on arthropods, are poorly understood. We examined the impact of silver wattle, Acacia dealbata, invasion on Coleoptera assemblages in the grasslands of the Drakensberg region, South Africa. Baited pitfall trap samples from uninvaded grassland and grassland sites invaded by A. dealbata were sorted into morphospecies. The composition of Coleoptera assemblages, attracted by cow dung, differed significantly between invaded and uninvaded grassland habitats. Coleoptera richness and especially abundance values were found to be significantly lower in the invaded stands compared to grassland. Mean body size showed significant differences when compared between the two habitat treatments. The invaded sites exhibited a marked decline in the larger and less abundant species. Also, parataxonomic units unique to the grassland were, in general, characterized by larger body sizes compared to those unique to the invaded sites. Clearly, A. dealbata invasion of grasslands has substantial negative impacts on Coleoptera diversity.