The effects of W plus PAR or PAR irradiation only on the cytosolic calcium concentration of the heterocysts of a filamentous cyanobacterium Anabaena sp. have been studied using the fluorescence microscopy technique and Calcium Crimson as a calcium indicator. After W + PAR irradiation a pronounced calcium signal, indicating an increase in the cytosolic calcium concentration, was observed in heterocysts in comparison to control cultures irradiated with PAR only. No increase in the calcium signal could be observed after UV + PAR irradiation of cells in Cat' free medium, but shortly after washing the cells in tap water the calcium signal reappeared in the heterocysts. This indicates that the signal is due to the uptake of external Cat+. Similarly, no increase in the calcium signal could be recorded in the heterocysts when the cells were irradiated with UV in the presence of lanthanum, which is known to block calcium channels. Washing the cells in tap water restored the calcium signal, indicating that the uptake of calcium is mediated by specific membrane channels, the opening of which is triggered by IN.