Interspecies competition is considered an important effector of community structure in ecosystems. Interspecies interactions may change due to changing environmental factors, including exogenous organic pollutants. In this paper, we measured the change in population density, based on the logistical growth model, of Microcystis aeruginosa and Scenedesmus obliquus when they were cultivated in single-species and mixedspecies cultures, under the stress of the polycyclic aromatic hydrocarbon (PAH) phenanthrene. The singlespecies culture experiment showed that S. obliquus could tolerate greater phenanthrene stress than could M. aeruginosa, and exhibited hormesis when the concentration of phenanthrene was 0.0625 mg/l. In the mixedspecies culture experiment, the toxicity of phenanthrene on the two algae changed. In the 0.0625 mg/l and 0.25 mg/l treatments, the population density of S. obliquus increased, whereas the population density of M. aeruginosa in each group decreased. Finally, the influence of different phenanthrene concentrations on the interspecies competition was evaluated.