The growth, photosynthetic characteristics, and competitive ability of three algal strains were investigated under different doses of ultraviolet-B (UVB) radiation (0, 0.285, and 0.372 W · m−2). The organisms were the toxic bloom-forming cyanobacterium Microcystis aeruginosa FACHB 912, nontoxic M. aeruginosa FACHB 469, and the green microalga Chlamydomonas microsphaera FACHB 52. In monocultures, the growth of all three strains was inhibited by UVB. In mixed cultures, enhanced UVB radiation resulted in decreased percentages of the two M. aeruginosa strains (19%–22% decrease on d 12 of the competition experiment). UVB radiation resulted in increased contents of chlorophyll a, b, and carotenoids (CAR) in C. microsphaera, and decreased contents of allophycocyanin (APC) or phycocyanin in the two Microcystis strains. All three strains showed increased levels of UVabsorbing compounds and intracellular reactive oxygen species under 0.372 W · m−2 UVB radiation, and decreased light compensation points, dark respiratory rates, and maximal quantum efficiency of PSII. After a 20 h recovery, the photosynthetic oxygen evolution of C. microsphaera was restored to its maximum value, but that of Microcystis strains continued to decrease. Nonphotochemical quenching was increased by UVB radiation in C. microsphaera, but was unaffected in the two M. aeruginosa strains. Our results indicated that C. microsphaera has a competitive advantage relative to Microcystis during exposure to UVB irradiation.