For hazard assessment of NPs quantitative nanoecotoxicological data are required. The objective of this review was to evaluate the currently existing literature data on toxicity (L(E)C50 values) of synthetic NPs in environmentally relevant species in order to: (i) identify tentatively most harmful NPs and most sensitive organism groups, and (ii) to provide relevant ecotoxicological information for further risk assessment. The focus was set on selected synthetic NPs (nano TiO(2), nano ZnO, nano CuO, nano Ag, SWCNTs, MWCNs and C60-fullerenes) and organism groups representing main food-chain levels (bacteria, algae, crustaceans, ciliates, fish, yeasts and nematodes). Altogether 77 effect values were found, mostly for nano TiO(2) (31%), C60 (18%), nano ZnO (17%), nano Ag (13%), SWCNTs and nano CuO (both 9%). Only 3% of the available quantitative ecotoxicological information concerned MWCNTs. Organism-wise, 33% of the data concerned crustaceans, 27% bacteria, 14% algae and 13% fish. For all organism groups studied, solubility of CuO- and ZnO-NPs was a key factor in their aquatic toxicity. On the basis of the 34 median L(E)C50 values derived from 77 individual values, NPs were ranked according to their lowest median L(E)C50 value for the above described organism groups: the most harmful were nano Ag and nano ZnO that were classified "extremely toxic", (L(E)C50<0.1mg/l), followed by C60 fullerenes and nano CuO that were classified "very toxic", (L(E)C50 0.1-1mg/l). SWCNTs and MWCNTs were classified "toxic" (L(E)C50 1-10mg/l). Nano TiO(2) was classified as "harmful", (L(E)C50 10-100mg/l). Throughout, algae and crustaceans (daphnids) were most sensitive and thus probably most vulnerable organism groups in aquatic exposure to NPs. Very low L(E)C50 values should deserve thorough attention of environmental risk assessors for evaluation of the potential adverse effects of synthetic NPs on ecosystems. As the quantitative nanoecotoxicological data are still rare, further studies are needed.