It was found that the concentration of Zn²⁺< at about 1.0μm0l/L was essential to keep optimal growth of Anabaena azotica Ley and also important to keep a quite high value of photosynthetic oxygen evolution rate, chlorophyll fluorescence (Fv/Fm) level and carbonic anhydrase activity, whereas the relative high concentration of Zn²⁺(≥5.Oμm) would inhibit the growth of A.azotica Ley, decrease its photosynthetic oxygen evolution rate, chlorophyll fluorescence and CA activity.The study of the absorption spectra of the phycobilisome-thy-lakoid membrane complex and the extraction of phycocyanin indicate that Zn²⁺ may affect the photosynthesis by its effect on the phycobilisome especially the phycocyanin.Since CA is a Zn²⁺-containing metalloenzyme and may provide a high concentration of CO₂ to Rubisco in the process of CO₂ fixation, it is suggested that Zn²⁺ may affect the photosynthesis by acting on CA.