Abstract.1. Estimates of worker populations ofFormica lugubrisZett. nests in Irish plantation woods were obtained using capture—mark—recapture methods. Forager numbers were estimated by paint‐marking individuals on the foraging routes and ‘recapturing’ by traffic counts. Colony‐size was estimated using Stradling's (1970)32P‐radiolabelling technique. Worker brood was estimated by excavation and direct count.2. Forager numbers ranged from 6906 to 64 686 (eleven nests), colony‐size ranged from 9797 to 71 052 (five nests) and worker brood ranged from 9809 to 16 269 (four nests).3. Forager number was highly correlated with the average traffic on individual routes and the nest forager populations declined rapidly in>30