In the first year's data of the 10th Finnish National Forest Inventory (2005–2008) 533 active and 178 abandoned ant mounds were found yielding 4.2 mounds ha^-1. We found 11 species: Formica rufa, F. polyctena, F. aquilonia, F. lugubris, F. pratensis, F. exsecta, F. fennica, F. pressilabris, F. forsslundi, F. suecica and F. uralensis. Five species occurred throughout the country, and four were restricted to the south. We found species-specific associations with either mineral soils or mires, with forest site type and with tree-canopy openness. Low fertility decreased the occurrence of polygynous species with large worker force, and sun-exposition was favourable for species with smaller colonies presumably because only large worker force enables metabolic thermoregulation of nests. Forest fragmentation and increased amount of edge habitats favourable for colony founding have presumably increased nest density whereas drainage of mires has reduced the amount of habitat of three species since the 1950s.