Abstract: In the colonies of Nostoc sphaeroides kütz.which suffered salt stress, the efficiency of excitation capture by open photosystem II (PSII) reaction centers (expressed as Fv/Fm) decreased with increased salinity.When treated with high salinities, the algal colonies lowered the Fv/Fm after prolonged treatment.The exogenous proline had little function in preventing the lowering of Fv/Fm on the salt stressed colonies.Meanwhile, the endogenous proline contents of the colonies didn' t increase with the increased salinities, whereas malondialdehyde (MDA) increased when the salt stress became more serious.In addition, the graphes of the SOD activities in the colonies have a typical "V" shape with treating salinities, presumably the changing was related to the real stresses that the colonies experienced .