This paper reports toxic effects and bioaccumulation factors of organometallic compounds, mainly organotin species, on algae. We selected two species of microalgae as test algae: one was Scenedesmus obliquus as a representative of fresh-water algae, the other Dunaliella salina and Dunaliella viridis, to represent commonly mixed algae which exist abundantly in Tianjin Harbor, People's Republic of China. For comparison, Chlorella vulgaris was also used in this study. The toxic effect of ten organometallic compounds on the freshwater alga, S. obliquus, was investigated. The ten compounds were (as chlorides) tributyltin (TBT); triphenyltin (TPT); trimethyltin (TMT); dibutyltin (DBT); diphenyltin (DPT); dimethyltin (DMT); trimethyl-lead acetate (TML); dimethyl-arsine (DMA) and two new mixed-alkyltin pesticides, dicyclohexylmethylitin acetate (Cy2MTA) and dicyclohexylmethyltin isobutyrate (Cy2MTB). The order of toxicity of these compounds in fresh-water algae, S. obliquus, was TBT>TPT>DBT> Cy2MTA=TML> Cy2MTB>DPT>TMT>DMA>DMT, according to 96 h EC50 values attained. The ten toxicants were divided into three groups according to the sequence of their toxicities; (a) TBT, TPT; (b) DBT, Cy2MTA, TML, Cy2MTB; (c) DPT, TMT, DMA, DMT. In each group the EC50 values of each compound were quite similar. The difference of EC50 values between two vicinal groups was approximately one order of magnitude. The bioconcentration factor (BCF) of TBT and TPT compared with water in the freshwater alga S. obliquus was >3.32 × 105 and 1.14 × 105, respectively. The BCF of the marine mixed algae was >3.48 × 105. The marine microalga, Chlorella vulgaris, was adaptable to TBT at lower concentration. TBT at high concentration only inhibited the growth of S. obliquus, but it could cause chlorosis anddisintegration of D. salina and D. viridis. Resistance to toxicity of algae against TBT appears in order as follows: C. vulgaris>S. obliquus>D. salina and D. viridis. TBT was metabolized by algae to a less toxic product, DBT. The existence of algal cells accelerated the concentration reduction of TBT. The toxic mechanism of TBT was also studied.