Abstract Photosynthesis of a cultured benthic blue-green alga Tolypothrix sp. was measured under light and temperature conditions using oxygen light-dark bottle method and ¹⁴C light-dark bottle method. Respiratory rates of the alga were also measured at different temperatures. Photosynthetic rates increased with increasing temperatures. Respiratory rates did not show significant differences among different temperatures. Photosynthetic rates were almost the same under light intensities lower than 400μEm^-2s^-1.Comparing the oxygen and ¹⁴C method, the photosynthetic quotient Q (moles O₂ per moles C) of Tolypothrix sp. in these experiments was about 1.11. The method for the measurement of photosynthesis of such kind of benthic alga was improved in this paper.