AbstractPhotosynthetic properties of high temperature (38 °C) grown (HTG)1and low temperature (25 °C) grown (LTG) cells ofAnacystiswere examined. The results revealed several characteristic changes in the HTG cells which included light absorption property of the pigments, DCMU binding affinity of the photosynthetic apparatus and photosynthetic yield. There was a time course dependent heat shock (47 ± 1 °C) induced inhibition of the O2evolution in both the types of cells. However, the response of the HTG cells was sluggish as compared to the LTG cells. Unlike the LTG cells, the heat shock treatment (47 ± 1 °C, 5 min) of the HTG cells did not result into a change in the pigment absorption spectra. The rise kinetics of fluorescence induction curve of chlorophyll a in the presence of DCMU and DCMU + NH2OH suggested for heat‐induced damage to the oxidizing side of photosystem II in both the HTG and LTG cells. Further, results on the Hill activity in the heat shocked HTG cells showed that the exogenous electron donors DPC and NH2OH not only retrieved the heat shock induced inactivation of the Hill activity, but stimulated the rate of activity. On the other hand, the rate of Hill activity in the heat shocked LTG cells was restored by only NH2OH an electron donor to PS II reaction centre, while DPC‐an electron donor to water oxidation complex was ineffective against the heat shock induced inactivation of the electron transport. These results, together, suggested for the presence of less number of heat susceptible sites in the HTG cells than that in the LTG cells, which might be accounting for the high degree of thermostability to the former type