Effects of chilling treatment on the photosynthetic activities and the light-absorption and fluorescence spectra were investigated in intact cells of the blue-green alga Anacystis nidulans that were grown at different temperatures. When the algal cells grown at 38 C were treated at 0 C for 10 minutes, the photosynthesis and the Hill reaction with 1,4-benzoquinone were significantly inactivated and the light-absorption spectrum of carotenoids was modified. These parameters showed very similar temperature dependencies in the chilling susceptibility and the temperature regions critical for the susceptibility depended on the growth temperature. The midpoint values for the critical temperature regions were 4, 6, and 12 C in cells grown at 28, 33, and 38 C, respectively. It is proposed that a common mechanism would underlie the chilling susceptibility of the photosynthesis, the Hill reaction, and the carotenoid absorption spectrum. The decoupling of excitation transfer from allophycocyanin to chlorophyll a at the chilling temperatures occurred very slowly and is attributed to a somewhat different mechanism of the chilling susceptibility.