Thesis: Tritium is a radioactive isotope of hydogen and is present in all environmental compartments. It can exist under the form of tritiated thymidine, and this form was selected in this work in order to better characterize its toxic effects. Its internalization and toxicity have been studied on the zebrafish, Danio rerio. This biological model has been chosen as it allows the comparison between the results obtained in this work and in previous ones, but also because of its various experimental advantages, as its rapid development and the transparency of its embryos. Exposure of zebrafish embryos and larvae to an absorbed dose rate as low as 22 µGy/h led to its internalization in both the organism and the DNA. The development was temporarily slowed down after exposure to 22 and 170 µGy/h, while the hatching was delayed after exposure to 170 and 470 µGy/h. Exposure of the embryos and larvae to 470 µGy/h for 24 and 72 hours led to, respectively, an increase in the embryonic activity and to a decrease in the larval cardiac rhythm. The ROS (reactive oxygen species) production increased after exposure to 170 µGy/h for 96 hours, and DNA damage were induced after exposures to all selected absorbed dose rates for 24 and 96 hours. A surexpression of various genes was also observed after exposure to 22 µGy/h for 24 hours. Micro- and nanodosimetry simulations confirmed the effect of the cell volume on the energy depositing processes, while also highlighting the formation of a majority of SSB (single strand breaks) and of indirect damage. These experiments showed that various differences exist between the toxicity of aqueous tritium, organically bound tritium and cesium (137Cs), while leading to a better understanding of the toxicity mechanisms arising from the exposure to tritiated thymidine.