Information on tungsten ecotoxicology is very limited. Moreover, it is an even larger lack of information linking tungsten environmental speciation and environmental effects. Soluble tungsten compounds can be released into the environment after corrosion and dissolution of tungsten containing alloys and minerals. Tungsten aquatic chemistry is quite complicated; tungsten anions may polymerize (depending upon concentration and aquatic geochemistry) in aquatic systems at environmentally relevant pHs. However, our present knowledge of tungsten toxicological effects is entirely based on the data obtained in experiments with monotungstates. Here we report experimental results demonstrating that polytungstates are significantly more toxic than monotungstates. Toxicological effects of sodium tungstate and metatungstate have been examined in the standard OECD toxicological tests for Daphnia, algae and earthworms. LD50 and EC50 values for sodium tungstate are more than 10 times larger than those for sodium metatungstate, when expressed in terms of tungsten concentration. This difference increases 12-fold more when LD50 and EC50 are expressed in terms of molecular concentrations. These toxicity patterns have also been observed in the experiment with laboratory aquatic ecosystems. At the ecosystem level, mono- and polytungstates also affect the nitrogen cycle. These findings stress the need for further studies of both environmental speciation of tungsten and toxicity of different tungsten compounds.