Acid precipitation from natural and human activities can lead to different extents of acidification in different freshwater ecosystems,with noticeable changes in the carbonate system and aquatic chemistry. For example,acidification is known to decrease the ionic concentrations of magnesium and calcium,but enhance aluminum concentrations. The changes to chemicals associated with freshwater acidification could induce physiological disturbance to most aquatic organisms, which would then need to spend additional energy to cope with acidic stress. Phytoplankton species with different sensitivities to acid stress may have differential physiological responses; thus, acid stress may lead to altered community structure and species richness. Alternatively, UV-B (280-320 nm) irradiance has been increasing due to stratospheric ozone depletion, which would also impact freshwater phytoplankton. Meanwhile, significantly decreased dissolved organic matter as a result of acid rain may further enhance the penetration of solar UV radiation (UVR, 280400 nm) in the water column due to decreased light attenuation. Therefore, freshwater acidification and solar UVR may interact to influence phytoplankton physiology. This study examined the physiological responses of a freshwater bloom forming cyanobacterium, Microcystis aeruginosa, to acidification and UVR. Two strains were used for the experiments; a toxigenic strain (FACHB-905) and a non-toxigenic strain (FACHB-469). Microcystis aeruginosa cells were grown under simulated acid rain (with a mole ratio of H_2SO_4:HNO_3 = 8:1) under a solar simulator with and without UVR,to observe whether photosynthetic performance and sensitivity to UV irradiance differed between the two strains. Three pH levels were used, pH 7.10 (regular BG11+N medium),5.65, and 4.50, with the latter two pH levels reflecting acidification conditions. Two solar radiation treatments were carried out, including:photosynthetically active radiation alone (PAR, 400700 nm) and PAR+ UV-A+ UV-B (PAB, 295-700 nm). Our results indicate that the mean cell size and cell volume of the 905 strain was significantly higher than those of the 469 strain in all pH treatments. Acidification treatment significantly lowered mean cell size and volume, but enhanced the chlorophyll a content in both strains. Under the lowest pH (4.50), higher mortality rates were observed for both strains,with decreased effective quantum yield. The inhibition of the growth rate was more pronounced in the non-toxigenic strain. The effective quantum yield declined with decreasing pH level when cells were exposed to high levels of PAR and PAR+UVR. UV-induced inhibition of effective quantum yield was much higher in the non-toxigenic strain compared to the toxigenic strain. This differential response may be attributed to different contents to UV absorbing compounds and carotenoids. Under the global climate change scenario, increasing aquatic acidification and UV irradiance might decrease the competence of the non-toxigenic strain over the toxigenic one.