A time-dependent loss of Photosystem II (PS II) activity seen in Anacystis nidulans grown without Ca(2+) was paralleled by a loss in chlorophyll (Chl) a fluorescence of variable yield which reflects inhibition of 'Q' reduction and of state changes. Both inhibitions were fully reversed by the addition of Ca(2+) to the growth medium. The lack of state changes in Ca(2+)-depleted cells was confirmed in 77 K fluorescence difference spectra of light versus dark-adapted cells.Absorption spectra of control and of Ca(2+)-depleted cells were identical whether measured at room temperature or at 77 K. Fluorescence emission spectra measured at 39°C (cell growth temperature) demonstrated higher yields in Ca(2+)-depleted cells compared to controls. Fluorescence emission spectra at 77 K also produced higher yields in Ca(2+)-depleted cells but the increased fluorescence at this temperature occurred principally at 683 nm. The increased relative fluorescence yield in Ca(2+)-depleted samples results from light absorbed by phycocyanin (PC), but not from light absorbed almost exclusively by Chl. The 683 run fluorescence peak probably represents increased allophycocyanin (APC) emission as intact phycobilisomes become energetically disassociated from the photosynthetic apparatus. This inferred disassociation occurred only after PSII activity was mostly inhibited in Ca(2+)-depleted cells, and was not fully reversible.