Tannery wastewater contains large quantities of organic and inorganic compounds, including toxic substances such as sulfides and chromium salts. The evaluation of wastewater quality in Chile nowadays is based on chemical specific measurements and toxicity tests. The goal of this research was to characterize tannery wastewater and to relate its physical/chemical parameters with its acute toxicity effect on Daphnia pulex. To distinguish the most important toxic compounds, physical/chemical techniques were applied to a grab sample of a final effluent based on the Phase I toxicity identification evaluation (TIE) procedure. In addition, the toxicity of a beamhouse effluent after an activated sludge reactor treatment was investigated on Daphnia magna (introduced species) and D. pulex (native species). Effluent from different tannery processes (soaking, beamhouse, tanning and final) demonstrated high values of chemical organic demand (COD; 2840-27 600 mg L-1), chloride (1813-16 500 mg L-1), sulfate (230-35 200 mg L-1), and total solids (8600-87 100 mg L-1). All effluents showed extremely toxic effects on D. pulex, with 24-h mean lethal values (LC50) ranging from 0.36% to 3.61%. The Phase I TIE profile showed that toxicity was significantly reduced by air stripping, filtration, and a cationic exchange resin, with toxicity reductions ranging between 46% and 76%. The aerobically treated beamhouse effluent showed significantly less toxicity for both species (43%-74%). The chemical parameters demonstrated that the remaining toxicity of the treated beamhouse effluent was associated with its ammonia (120 mg N-NH3 L-1) and chloride (11 300 mg CI- L-1) contents. © 2003 Wiley Periodicals, Inc. Environ Toxicol.