Chlorate-resistant (ChlR) mutants SM1' and SMZ5 of the cyanobacterium Spirulinu mujor were produced by N-methyl-N'-nitro-N-nitrosoguanidine (NTG) ') mutagenesis. Strain SM' ' showed nitrate reductase (NR) and glutamine syntbetase (GS) activities equal to the SMP (parent) whereas SMZ5 had about 45% NR and 78% GS as compared to SMP. However, the uptake of nitrate or ammonia was almost equal in SMP, SM" and SMZ5. ChlR strains differed physiologically between themselves, SM" being blocked in uptake system of chlorate (Chl up-) and was not defective in NR or GS, while SMZ5 showed chlorate uptake (Chl up') and was defective in NR (NRdef) and GS (GSdef). Thus strain SM" resembles SMP except that the latter is chlorate sensitive. This is the first report of defective NR in any filamentous, non-heterocystous, non-nitrogen fixing cyanobacterium. Defective chlorate uptake not accompanied by defective NO; uptake has also been observed in strain SM". Chlorate (C10;) is a structural analogue of nitrate. It is reduced to the chlorite ((210;) ion by assimilatory NR . Chlorite is highly toxic to those organisms which use nitrate (NO;) as nitrogen source. Thus, chlorate-incubated cells would result in production of toxic chlorite by NR, ultimately causing death. Accordingly, ChlR strains would be expected to be defective in: NO; uptake, NR, or both. It is difficult to distinguish physiologically between NO; transport and reductase systems, and misleading conclusions may be drawn about the regulatory properties of one system independent of the other. Cyanobacterial mutants resistant to chlorate and defective in transport and/or reduction of NO; have been isolated (SINGH and SONIE 1977, PANDEY and, none of which was able to utilize NO, despite retaining the ability to grow on molecular N, or NH;. Studies on ChlR mutants of the green alga Chlamydomonas reinhardii (NICHOLS et ul. 1978) and the fungus Aspergillus sp. have suggested that NO; uptake and reductase systems are distinct. Spirulina contains the enzyme ferredoxin sulfite reductase which is analogous to nitrite reductase (NIR) but whether or not these enzymes may be involved in detoxification of chlorate is not known. We have attempted to study some of these aspects in Spirulina mujor. ')