Fluoranthene is one of the polynuclear aromatic hydrocarbons with four benzene rings. Because of its toxicity,mutagenicity, and carcinogenicity, fluoranthene is on the black lists of 129 and 68 priority pollutants established by US Environmental Protection Agency and the Peoples Republic of China, respectively. In recent years, the amount of fluoranthene in the aquatic environment has been increasing with increases in anthropogenic discharge. Based on the biological investigation of tidal water in the Futian mangrove, Cyclotella caspia was selected as the dominant algal species to determine the toxicity of fluoranthene towards C. caspia alga and to investigate the biodegradation of fluoranthene by C. caspia under pure culture. The toxicity experiment showed that the 96-h EC50 vaiue for fluoranthene was 0.2 mg/mL. Four parameters, namely C. caspia algal growth rate,chlorophyll (Chi) a content, cell morphology, and superoxide dismutase (SOD) activity, were chosen as indices of toxicity and were measured at 6 d (144 h). The results showed that: (i) the toxicity of fluoranthene towards C.caspia alga was obvious; (ii) C. caspia algal growth rate and Chi a content decreased with increasing concentrations of fluoranthene; and (iii) the rate of cell deformation and SOD activity increased with increasing concentrations of fluoranthene. The biodegradation experiment showed that: (i) the rate of physical degradation of fluoranthene was only 5.86%; (ii) the rate of biodegradation of fluoranthene on the 1st and 6th days (i.e. at 24 and 144 h) was approximately 35% and 85%, respectively; and (iii) the biodegradation capability of C. caspia alga towards fluoranthene was high. It is suggested that further investigations on the toxicity of fluoranthene towards algae, as well as on algal biodegradation mechanisms, are of great importance to use C. caspia as a biological treatment species in an organic wastewater treatment system.