Turfs of 'Park' Kentucky bluegrass reestablished in the greenhouse and inoculated withKlebsiella pneumoniae(W6) showed significantly increased nitrogen fixation (acetylene reduction) compared with control turfs. Mean ethylene production rates per pot were 368 nmol h−1forK.pneumoniaetreated turfs, 55 nmol h−1for heat-killedK.pneumoniaetreated turfs, and 44 nmol h−1for untreated turfs. Calculated lag periods before activity was observed were generally very short (less than 1 h).When 'Park' Kentucky bluegrass was grown from seed on soil-less medium of Turface, a fired aggregate clay, inoculation withK.pneumoniae(W6) resulted in 9 of 11 turfs showing nitrogenase activity (mean ethylene producion rate per pot was 195 nmol h−1). Only 3 of 11 turfs treated with heat-killedK.pneumoniaeshowed any activity and their mean rate of ethylene production (40 nmol h−1per pot) was significantly lower than that for turfs treated withK.pneumoniae.Using the 'Park'–Turface soil-less model system it was shown that acetylene reducing activity was (i) root associated, (ii) generally highest at a depth of 1–4 cm below the surface, (iii) enhanced by washing excised roots, and (iv) inhibited by surface sterilization of excised roots.Klebsiella pneumoniaewas recovered from Turface and roots showing acetylene reducing activity.