Abstract: Single cells of Spirulina platensis were made by treating the initial strain(Sp)NS-90020 with Ultrasonic Homogenizer, then the mutant, SP-ALT₁, were selected afterinducing these single cells with NTG.And their general morphology,physiological features and biochemical composition were studied comparatively.There are some morphological differences between the initiall strain and the mutant.The filaments of the mutant show much shorter lengh and fewer numbers of the helix than of the initial strain.The temperature curves of photosynthesis showed that the optimum temperature of oxygen-evolution of SP-ALT₁, was lower than that of the initial strain.When they weretreated at 4'C,the mutant could endure much lower temperature than the initial strain.These showed that SP-ALT₁, was a middle temperature strain.In addition,the protein in SP-ALT₁, increased by 9.5% compared with the initial strain.