An ecological hazard assessment was performed for 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin in the aquatic environment, taking into account four different classes of species which differ with respect to their positions in aquatic food webs: invertebrates, fish, birds preying on aquatic organisms, and mammals preying on aquatic organisms. The most sensitive toxicological endpoints for each class were selected from the literature. These endpoints were extrapolated to no effect concentrations (NECs) in water with a two-level food chain model as indicator for secondary poisoning. From the results it is concluded that fish-eating birds and mammals experience larger hazards from dioxins and related compounds than fish. The limited data available appear to indicate that invertebrates are rather insensitive to dioxins. The study indicates that assessment of secondary poisoning affects environmental quality criteria setting and risk assessment.