The effect of tributyltin chloride (TBTC) and triphenyltin chloride (TPTC) on photosynthesis of pure cultures of P. sp. and D. zhanjiangensis were tested. The two species of marine microalgae were cultured in a range of concentration of TBTC and TPTC (0.2, 0.4, 0.6, 0.8, 1.0μg/L) for 24h, 48h and 72h respectively. Different effects were observed within 24h at 0.2μg/L of TBTC and TPTC, and photosynthesis and growth were markedly reduced after culture of the species in >= 0.4μg/L of the two chlorids.