Heat stress caused rapid and severe inhibition of photosynthesis and nitrate reduction in nitrate-supplemented cultures of the cyanobacterium Anabaena sp. strain L-31, compared to nitrogen-fixing cultures. Anabaena strains harbour two hsp60 family genes, groEL and cpn60, respectively encoding the 59 kDa GroEL and 61 kDa Cpn60 chaperonin proteins. Of these two Hsp60 chaperonins, GroEL was strongly induced during heat stress, irrespective of the nitrogen status of the cultures, but Cpn60 was rapidly repressed and degraded in heat-stressed nitrate or ammonium-supplemented cultures. The recovery of photosynthesis, nitrate assimilation and growth in heat-stressed, nitrate-supplemented cultures were preceded by resynthesis and restoration of cellular Cpn60 levels. Glutamine synthetase activity, although adversely affected by prolonged heat stress, was not dependent on either the nitrogen status or Cpn60 levels during heat stress. Overexpression of the Cpn60 protein in the closely related Anabaena sp. strain PCC7120 conferred significant protection from heat stress to growth, photosynthesis and nitrate reduction in the recombinant strain. The data favour a role for Cpn60 in carbon and nitrogen assimilation in Anabaena.