The heat shock response of three cyanobacterial strains, Anabaena sp. Strain PCC (Paris Culture Collection) 7120, Plectonema boryanum Strain PCC 6306, and Synechococcus sp. Strain PCC 7942, was characterized by polyacrylamide gel electrophoresis. Anabaena produced 33 heat shock proteins, P. boryanum 35 proteins, and Synechococcus 19 proteins. The rapid response to heat shock was consistent for all three strains, although the number of time-dependent proteins varied. All strains developed thermotolerance when first pretreated with a sublethal heat shock and then challenged with a previously lethal temperature. A 30-min 30-degrees-C incubation was required between the heat shock and challenge for Synechococcus, but not for Anabaena and P. boryanum. Synechococcus cells required a higher challenge temperature (51-degrees vs. 49-degrees-C) than the other two strains to destroy control cells that were not pretreated with a heat shock.