The morphology and nitrogenase activity (ARA) of Gloeotrichia pisum Thur. growing on deepwater rice were studied in situ at Sonargaon, Bangladesh, and compared with results from laboratory studies. The alga showed obvious similarities and dissimilarities. Hairs, gas vacuoles and akinetes, which were found in the field, were not found In the laboratory, even though the influences of quantity and quality of light flux and nutrient deficiency were tested. The response of nitrogenase to changes in light flux was rapid. A reduction in flux brought about similar results in the field and In the laboratory. Diel changes in nitrogenaee activity showed a maximum at mid-day In the presence of maximum light flux, but considerable activity (3.7% of dally total) took place at night. The activity In a medium free of combined carbon by bacteria free G. pisum indicates that the rate and duration of nitrogenase activity in the dark depend on the preceding light conditions. In batch culture, higher nitrogenase activity was observed when dark-grown alga was reilluminated, than the maximum activity ever found under continuous Illumination. The variation of nitrogenase activity in batch culture was studied in relation to the growth characteristics and developmental stages of the alga. Maximum activity (1.4 nmol C(_2)H(_4) µg ch1 a(^-1) min(^-1)), was observed after about one day of growth under continuous light. During this period, juvenile filaments were abundant (hence maximum heterocyst frequency), cyanophycin granules lacking and cultures had the lowest ch1 a : d. wt ratio. Heterocyst differentiation occurred between 14 and 24 h of growth and cell division was observed only after heterocyst differentiation. It appears that the juvenile filament is the most active nitrogen fixer during the life cycle of the alga. Reasons for low nitrogenase activity of about 0.352 nmol C(_2)H(_4) µg ch1 a(^-1) min (^-1) in the field have been discussed. It is estimated from diel changes in N(_2)-fixing activity and based on ARA of 77.4 nmol C(_2)H(_4) tiller(^-) min(^-1) and(^2) using some approximations, that G. pisum contributed about 4 kg N ha(^-1) season" in deepwater rice fields at Sonargaon in 1983.A brief morphological study was also Included. In the field, rapid colonization was observed on freshly submerged rice culms. Colonies contained Intersheath spaces on their periphery and exhibited zonation of filaments. In the larger ones. The mechanism of the formation of a radiating colony in the laboratory has been described and discussed. Differentiation of a hormogonium into a filament was studied in batch culture. The basal youngest cell differentiated into a heterocyst, whilst the rest of cells divided repeatedly, forming several groups of cells in a row. Subsequently most of these groups of cells were liberated successively as hormogonia.