Abstract: The investigation deals,with the nitrogen fixation of some blue-green algae isolated from the soils of rice fields.Samples of algae-bearing soils were collected from rice fields in the provinces of Hupeh,Hunan and Kiangsi.Uni-algal cultures were made first on media enriched with nitrogenous compounds to encourage vigorous growth.The uni-algal cultures were then subcul- tured onto nitrogen-free media,which effected a preliminary separation of those algae which could thrive in the absence of pre-formed nitrogenous compounds,from those which could not.The algae which showed a prolific growth on the nitrogen-free medium may be considered either to be capable of“fixing”(assimilating)atmospheric nitrogen,or that their nitrogen supply is coming from Azotobacter present in the culture.Our next step was to obtain bacteria-flee unialgal cultures. Two bacteriostatic methods were used:ultra-violet radiation from 300-watt quartz lamps,and treatment with streptomycin. Bacteriostatically treated uni-algal cultures were tested by inoculating sterile nitrogen-free media suitable for Azotobacter and allied organisms.Examination of these subcultures from the irradiated material showed that an intermittent radiation from our lamps of 5 minutes,repeated two or three times,was an effective bacteriostatic.Also,that Azotobacter and other micro-organisms were killed after treatment with 20 ppm.of streptomycin. The capacity for assimilating free nitrogen from the air was assessed by determining the quantity of nitrogenous compounds produced in the algal cells and in the medium,by means of the micro- Kjeldhal method.The nitrogen-fixing capacity has now been assessed on 4 isolated types of Blue- green Algae:HB 686(dnabaena azotica);HB 678(dnabaena azotica f.alpha);HB 670 (Anabaena variabilis forma);and HB 508 (Nostoc Linckia forma).The highest rate of fixation, amounting to 1.0146 rag.N/100 cc.of nitrogen-flee medium in 4 days,was attained by HB 686; the next,of 0.9382 mg.N,by HB 678,0.8614 nag.N,by HB 670;and 0.7592 mg.N,was produced under the same conditions,by HB 508.