Rice (IR26) was grown in Luisiana soil (pH 6) in 16 plots (8 treated with Ca and P prior to transplanting, the other 8 left untreated). N-fixing activity was assayed by acetylene reduction technique both in situ and in water culture. There was little difference in the N-fixing activity thus obtained for the different growth stages of the plant between both treatments. The daily average amount of N fixed was 6-8 g/ha which was much lower than that for the same rice variety grown in Maahas soil (pH 6.8) (55 g/ha). In general, the populations of heterotrophic bacteria, Azospirillum, and glucose-utilizing, aerobic N-fixers in the root zone of plants grown in Luisiana were lower than those in Maahas rice roots. Only 2-15% of the bacterial isolates from the histophere of Luisiana-grown rice plants were capable of both N-fixation and growth under 20% H gas as against 70-90% of the isolates from Maahas plots.