The changes in pigment composition, photosynthesis and PSII photochemistry were investigated in cells of Spirulina platensis adapted to salt stress (<0.75 M NaCl). A decrease in the phycocyanine/chlorophyll and no significant change in the carotenoid/chlorophyll ratio were observed in salt-adapted cells. Salt stress inhibited the apparent quantum efficiency of photosynthesis and PSII activity while stimulating PSI activity and dark respiration significantly. Salt stress also resulted in a decrease in overall activity of the electron transport chain, which could not be restored by diphenylcarbazide, an artificial electron donor to the reaction centres of PSII. Measurements of the polyphasic fluorescence rise in fluorescence transients including phases O, J, I and P showed that salt stress had no effect on the fluorescence yield at phase O but decreased the fluorescence yield at phases J, I and P. Analyses of the JIP test developed from the polyphasic rise of fluorescence transients showed that salt stress led to a decrease in both the maximum quantum efficiency of PSII photochemistry and the maximum quantum efficiency of electron transport beyond the primary quinone electron acceptor. However, salt stress induced no significant changes in the probability of transporting an electron beyond QA, the trapping flux per PSII reaction centre, or the electron transport flux per PSII reaction centre. A theoretical analysis of fluorescence parameters indicated a decrease in the rate constant of excitation energy trapping by PSII reaction centres. In addition, salt stress induced an increase in the complementary area above the fluorescence induction curve in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, suggesting an increase in the proportion of closed PSII reaction centres in salt-adapted cells. Based on these results, it is suggested that modifications in PSII photochemistry in salt-adapted Spirulina cells maintained a high conversion efficiency of excitation energy, such that no significant change was observed in either the trapping flux or the electron transport flux per PSII reaction centre.