Risk assessments from the European Union and the World Health Organization report values for acute and chronic toxicity of Cr(III) to Daphnia magna in the range of 0.6 mg/L to 111 mg/L and 0.047 mg/L to 3.4 mg/L, respectively. To understand whether factors other than the use of different test media and data reporting contribute to this variability, the authors tested the acute (48-h) and chronic (21-d) toxicities of Cr(III) to D. magna according to Organisation for Economic Co-operation and Development (OECD) methods. Filterable (0.45-µm) chromium concentrations were measured at 0 h, 6 h, 24 h, and 48 h, the latter value corresponding to the total duration of the acute tests and to the interval between medium renewals in chronic tests. In highly alkaline media (4.9 meq/L), Cr concentrations decreased rapidly below the analytical detection limit, and no toxicity was observed. In less alkaline media (approximately 0.8 meq/L), the decrease in filterable Cr concentrations was inversely proportional to the quantity of added Cr(III). The authors concluded that existing data likely underestimate the ecotoxicity of Cr(III) to D. magna. A reliable assessment of the hazard of Cr(III) to D. magna must consider that exposure concentrations can decrease markedly from the beginning to the end of a test and that medium alkalinity strongly influences the outcome of laboratory toxicity tests.