Nostoc ANTH is a filamentous, heterocystous cyanobacterium capable of N-2-fixation in the absence of combined nitrogen. A chlorate-resistant mutant (Clo-R) of Nostoc ANTH was isolated that differentiates heterocysts and fixes N-2 in the presence of nitrate, but not in the presence of nitrite or ammonium. The mutant lacks nitrate uptake and thereby also lacks induction of nitrate reductase activity by nitrate. However, this mutant is able to transport and assimilate nitrite, indicating that there is a transport system for nitrite that is distinct from that for the nitrate. The lack of inhibitory effect of nitrate on N-2-fixation was owing to lack of nitrate uptake and not to lack of enzymes for its assimilation (nitrate reductase and glutamine synthetase) or the lack of an ammonium transport system for retention of ammonia. The mutant has potential for use as a biofertilizer supplementing chemical nitrate fertilizer in rice fields, without N-2-fixation being adversely affected.