Organotins are widely used organometals in various agricultural and industrial purposes. After introduction of these chemicals to the aquatic environment, they are degraded by abiotic and biotic precesses. The triorganotin compounds are sequentially degraded to di-organotin, mono-organotin and then finally inorganic tin. Although the effects of trialkyltin an marine organisms have been intensively studied, little has been known on plankton as a producer of ecosystem. In this paper, the toxicities of dibutyltin (DBT), monobutyltin (MBT), diphenyltin (DPT), monophenyltin (MPT), trimethyltin (TMT) and dimethyltin (DMT) to rotifer Brachionus plicatilis were measured, and their potencies were compared based on 96 hr-$LC_{50}$ value. The results showed that DPT (13.8 ppb) was the highest toxic, which was followed by TMT (42.9), DBT (80.6), MPT (262.2), MBT and DMT (>1,000) in order. Thus, in tri- and diorganotins, the toxicity was observed phenyltins > butyltins > methyltins, and in mono-organotins phenyltins was more toxic than butyltins. Considering the order of 96 hr--$LC_{50}$ with octanol-water eoefficients ($K_{ow}$) in organotins together, it was considered that the toxicity of organotins seems to be related to the lipophilicity of the compounds.