The ozonation products of humic acids are toxicologically interesting for two reasons: by their actual toxicity, but also because of their complexing capacity vis-à-vis trace metals present in water which are, because of this fact, incompletely eliminated by potabilization techniques. Determination of the ultimate toxicity of ozonized humic acids is achieved with the aid of the medium term Daphnia magnatest. Statistical treatment of the two lethal parameters and of the four sublethal parameters have shown a toxicity for ozonized humic acids in solution, at a concentration of 100 mg−1.