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S U M M A R YNitrogenase activity in the rhizosphere of a grass, Paspalum notatum, and its associated soil was measured by the reduction of acetylene. Roots of the cultivar ' batatais' colonized by Azotobacter paspali, when taken from the soil, produced I to 32 nmol C,H,/g dry wt/h, whereas the cultivar 'pensacola', which is not colonized by A . paspali, produced less than 0.5 nmol/g/h. There was a lag of 12 to 24 h before maximum, linear, rates of acetylene reduction were reached. Activity was almost completely inhibited in air or in the absence of 0, and was greatest at around PO, 0.04 atm. Activity of soil cores containing plants with leaves attached was little affected by PO, and showed no lag. Soil-plant cores maintained in a 16 h day+8 h night showed no diurnal fluctuation in activity; as the dark period was extended, activity decreased but was restored on returning plants to the light. Roots and rhizomes had most activity, the soil very little and aerial parts none. Washing the roots removed less than half the activity. Disturbance of soil-plant cores decreased activity. The soil next to the root surface contained most A. paspali; more were associated with active plants than with less active plants. Sections of roots showed abundant bacteria adjacent to the root surface. Nitrogen fixation by the association was estimated to be up to 90 kg N/ha/annum.