Disinfection to protect human health occurs at drinking water and wastewater facilities through application of non-selective oxidants including chlorine.Oxidants also transform organic material and form disinfection by-products (DBPs),many of which are halogenated and cyto-and genotoxic.Only a handful of assays have been used to compare DBP toxicity,and researchers are unsure which DBP(s) drive the increased cancer risk associated with drinking chlorinated water.The most extensive data set employs an in vitro model cell,Chinese hamster ovary cells.Traditionally,most DBP research focuses on the threat to human health,but the effects on aquatic species exposed to DBPs in wastewater effluents remain ill defined.We present the developmental toxicity for 15 DBPs and a chlorinated wastewater to a model aquatic vertebrate,zebrafish.Mono-halogenated DBPs followed the in vivo toxicity rank order:acetamides ＞ acetic acids ＞ acetonitriles ～ nitrosamines,which agrees well with previously published mammalian in vitro data.Di-and tri-halogenated acetonitriles were more toxic than their mono-halogenated analogues,and bromine-and iodine-substituted DBPs tended to be more toxic than chlorinated analogues.No zebrafish development effects were observed after exposure to undiluted or non-concentrated,chlorinated wastewater.We find zebrafish development to be a viable in vivo altative or confirmatory assay to mammalian in vitro cell assays.