Abstract: The object of these experiments was to test the effect on rice yields of the inoculation of rice fields with blue-green algae,which are powerful nitrogen-fixers.In our investigations,three cultured types were selected for trials:HB 686(Anabaena azotica),HB 678(Anabaena azotica f.a.)and HB 670(Anabaena variabilis vat.tenuis).In one series they were inoculated into the soil for pot- cultures,and in another into the soil of an experimental rice field,both having controls. The results of our experiments are:the pot-culture inoculated HB 678 showed on increase in yield of 23.74%,and with HB 670 the increase was 18.73%.In the field trials inoculation with HB 686 showed an increased yield of 24.08% and HB 678 and increase of 17.79%,compared with the control.