SUMMARY (1) Ant-hills built by Lasius flavus (F.) occur frequently in permanent grasslands in southern Britain and bear a distinctive flora. (2) The vegetation of large ant-hills was compared with that of the surrounding grassland at thirteen chalk grassland and chalk heath sites in southern England. At each site, the percentage cover of each species was estimated on eight mounds and in eight associated pasture quadrats. (3) Most plant species showed consistent patterns in relation to ant-hills. (4) Annuals, particularly Arenaria serpyllifolia, were often confined to ant-hills. The perennial chamaephytes Cerastium holosteoides, Helianthemum chamaecistus and Thymus drucei were also significantly more abundant on the mounds (P < 005). (5) Of the species which were equally abundant on and around the ant-hills, some, like Agrostis stolonifera, Campanula rotundifolia, Festuca rubra and Galium verum, are stoloniferous and able to grow up through soil heaped onto the surface of the mounds by the ants, and others, such as Euphrasia officinalis agg., Gentianella amarella, Linum catharticum and Medicago lupulina, are short-lived and able to flower on the anthills. (6) The perennial herbs Carexflacca, Cirsium acaule, Filipendula vulgaris, Hieracium pilosella, Leontodon hispidus, Plantago lanceolata, P. media, Poterium sanguisorba, Succisa pratensis and Trifolium pratense, together with the tufted perennial grasses Dactylis glomerata, Helictotrichon pratense and Zerna erecta, were among the species which were significantly more abundant in the pasture than on the mounds (P< 0 05). (7) The differences in environment between ant-hills and the surrounding grassland are discussed in detail. Soil analyses showed no large or consistent differences in nutrient content between ant-hill and pasture soils. Compared with the surrounding pasture, the mounds have a shorter vegetation, greater dunging pressure from rabbits, and a drier soil with more bare patches, smaller structural aggregates, a lower bulk density and greater extremes of temperature.