This 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 L-' range. These congeners are also the most readily bioaccumulated.An aquatic toxicity threshold concentration of 0.01 1-0.038 ng L-', 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 concentrationllowest-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 appropriate. 0 1995 by John Wiley & Sons, Inc.