Abstract: Azolla imbricata and Azolla filieuloides were studied in regard to the nitrogenase- catalyzed reactions of C2H2 reduction and H2 production, employing gas chromatography; and photosynthetic CO2 uptake as well as simultaneous determinations of C2H2 reduction, photosynthesis and respiration in a closed system. Photosynthetic CO2 uptake and respiratory C02 production were determined using an infrared gas analyzer. These studies have indicated the following 1. Nitrogenase-catalyzed C2H2 reduction is largely light dependent. About 10,000 lux were required for saturation in A imbricata. A concentration of 10% C2H2 in the gas phase is saturating for C2H2 reduction and 1% CO inhibits C2H2 reduction with concomitant H2 production. 2. A determination of C2H2 reduction activity as a function of leaf age established a develop mental gradient in both A. imbricata and A. fgliculaides. In both species activity is negligible in the apex, increases markedly in progressively older leaves, plateaus, and decreases as leaves senesce. The developmental gradient of activity is much steeper in A. imbricate than in A. filiculoieds due to differences in their gross morphology. 3. Nitrogenase-eatalyzed H2 production in A. imbricata was not detectable under Ar but was appreciable under Ar containing 15% C2H2 and 2% CO. H2 production was also determined under the latter gas phase as a function of leaf. These studies implicate the occurrence of an uptake hydrogenase. 4. The photosynthetic compensation points in air are approximately 30 ppm CO2 for A. imbricata and A. filiculoides. 5. Simultaneous measurements of photosynthesis, respiration and C2H2 reduction in A. imbricata demonstrated the immediate dependence of nitrogenase on photosynthetically captured radiation for energy but an indirect dependence on CO2 fixation.