In chlordecone (CLD) contaminated soils of the French West Indies, if microbial remediation or a physicochemical remediation process, e.g., in situ chemical reduction, is implemented, concentrations of degradation byproducts, such as hydrochlordecones, are expected to increase in the ecosystems. To study their impact in mixtures with CLD, bioassays were carried out. They consisted in evaluating the regenerative capacity of hydra polyps, from a clone whose phylogenetic analysis con rmed that it belonged to the species Hydra vulgaris Pallas, 1766. Hydra gastric sections were exposed to CLD alone or CLD plus dechlorinated byproducts (CLD-BPs) for 96 h to assess regeneration. Based on chromatographic analysis, the CLD-BPs mix was composed of the 5-monohydrochlordecone isomer (CAS nomenclature), four dihydrochlordecone isomers and one trihydrochlordecone isomer representing 50%, 47% and 3% of the total chromatographic area, respectively. A total of 18 mixtures of CLD and CLD-BPs were tested. Six environmental concentrations of CLD (2.10 − 4 µM to 4.10 − 2 µM) and a similar range of CLD-BPs were used. Results from exposures to CLD alone showed: (i) a signi cant decrease in the regenerative capacity of hydra, except at the lowest concentration (2.10 − 4 µM), (ii) a concentrationindependent deleterious effect. The regeneration scores obtained after the exposure to the addition of CLD-BPs were not signi cantly different from those obtained after exposure to CLD alone. Using an experimental design, a modeling of the regeneration scores of hydra exposed to mixtures is proposed.Interpreted carefully, since they are limited to only one type of bioassay, the present results suggest that the situation in the aquatic environments should not become worse in terms of toxicity, if soil remediation programs resulting in the formation of hydrochlordecones are put in place.