The sewage contains toxic chemical compounds that secondary treatment plants do not eliminate, and chlorine is usually added for disinfection before discharge. Chlorine reacts with sewage compounds forming other toxic compounds. The objective of this study was to evaluate the toxicity of treated sewage from a secondary treatment plant using Danio rerio embryos. Three types of treated sewage were tested, one disinfected with chlorine gas (dCl2), another with sodium hypochlorite (dClO), and the plant discharge, which is a mixture of the two sewage disinfection methods (mCl) with a proportion of 70% of dCl2 and 30% of dClO. To estimate the median lethal dilution treated sewage was diluted with dechlorinated tap water at 20, 40, 60, and 80%. Two additional points were made with pure treated sewage and pure dechlorinated tap water. Embryos were exposed 1 h after fertilization (hpf) to 144 hpf. The dCl2 and the mCl were lethal at 60 and 72 hpf, respectively, while the dClO did not show lethality. The embryos exposed to the mCl showed heart failure and slower blood circulation. Those exposed to dCl2 showed teratogenic effects such as pericardial edema and spinal curvature, while those exposed to dClO presented malformations such as incomplete eye development and otolith formation absence.