Chlorate-resistant mutants of the filamentous cyanobacterium, Anabaena doliolum, were isolated by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)1 mutagenesis. Three classes of mutants were obtained that were altered either in the nitrate uptake activity or nitrate reductase enzyme activity or both. These results suggest that the genetic determinant of the uptake system was distinct from that of the reductase system. Uptake studies of nitrite and ammonium and rate of nitrite reductase activity in the mutants revealed that the nitrite and ammonium metabolisms were not affected by this mutation. Both nitrate and chlorate acted like a pair of antagonists, with nitrate protecting the growth against chlorate with increase in its concentration; similarly, increasing chlorate concentrations counteracted the growth-protective action of nitrate.