Abstract: We have previously reported that a significant rate, 10tq 15% as that of photosynthetic oxygen evolution, of aerobic hydrogen production had been observed in several rapidly-growing nitrogen-fixing strains of blue-algae (Cyanobacteria) under flow system. The contineously high aerobic hydrogen photoproduction mutants (Designated N9A and 18A, meaning the mutagenesis being performed on Nov.9 and 18, at Am. 1982) of Anabaena spp. Strain CA was induced by using mutagen N-methyl-N-nitro-N-nitrosoguanidine (NTG) in ASP-2 medium containing 100 nM Ni++, 1% water-washed agar and minus nitrogen source. The mutants were incubated in closed flack for 50 hs with the cells immobilized in agar During the incubation, the mutants aceumulated H2 at a quantity of 2 times (mutant N9A) and 6 times (mutant 18A) as much as that of the wildtype, and for mutant 18A, it was equal to 1.8% H2 in the gas phase (1% CO2 in air). Under the saturated light intensity of growing, the capacity of net aerobic hydrogen production for the wildtype was found to increase with the increasing light intensity, the reason was that H2 uptake was reduced as growth light intensity increased. However, there was no detectable H2 uptake in the mutants even they were grown under low light intensity. It suggests that the high light intensity may inhibit the activity of uptake hydrogenase, and this enzyme or the relevant electron transport may have been lost or damaged in the mutants.The mutants are not significantly different from the wildtype in growth rate in liquid medium, in chlorophyll a concentration and in the rates of C2H2 reduction and photosynthetic oxygen evolution. In wildtype, the addition of 50 to 100 nM Ni+2 to growth medium abolished net H2 photoproduction and increased the rate of dark H2 uptake to about 10 times, while in both mutants, H2 evolution was independent of Ni+2 in growth medium.The results suggest that these mutants, N9A and 18A, are the phenotypic Hup- mutants of nitrogen-fixing Cyanobacterium, Anabaena spp. strain CA.