A modified technique for measurement of nitrogen fixation by blue green algae and Azolla sp. using intact, moist soil cores was developed for use in field studies in which chemical fertilizers, blue green algae and Azolla biofertilizers had been used in association with a rice crop. This method involves collection of fresh and moist soil cores (0–30 mm) using a soil auger, incubation with 10% acetylene in airtight glass vials under field conditions for three hours, and measurement of the ethylene produced using a gas chromatograph. Experiments carried out over a period of three consecutive years revealed that the estimates of nitrogen fixation were comparable with data of other researchers obtained through the use of soil water columns, sophisticated chambers and automated sampling devices. This simplified technique, therefore, can provide rapid and reliable measurements of nitrogen fixation in field experiments.