AbstractThis paper reviews published studies on polychlorinated dibenzo‐p‐dioxins and polychlorinated dibenzofurans that are relevant to an assessment of their aquatic ecotoxicology. The available data suggest that laterally substituted congeners containing altogether 4,5, or 6 chlorine atoms are highly toxic, particularly to the early life stages of fish, with reported effect concentrations in the ng L1range. These congeners are also the most readily bioaccumulated.An aquatic toxicity threshold concentration of 0.011–0.038 ng L−1, applicable to natural ecosystems, was determined for 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (or for mixtures of congeners expressed as 2,3,7,8‐TCDD equivalents). This threshold corresponds to the no observed effect concentration/lowest‐observed effect concentration determined for mortality, growth, and behavioral effects seen in rainbow trout early life stages, exposed to 2,3,7,8‐TCDD in a flow‐through system over a 28‐day period, followed by 28 days of depuration.An investigation was also made into the potential for using mammalian‐derived toxic equivalency factors (TEFs) in aquatic ecosystems. This revealed that such an approach may underestimate the aquatic toxicity of some congeners, and that the use of organism‐specific TEFs may be more appropriat