Natural aquatic populations are at risk of an exposure to radionuclides emitting different types of radiation. Exposure of an organism to radionuclides leads to different effects that depend on different factors such as the radiation type, its cellular repartition, the activity, and the exposure duration leading to an absorbed dose. The subcellular characterization of the radionuclide distribution and its associated absorbed dose are then crucial to characterize the underlying mechanisms associated to each induced effect. Toxicity of beta radiations on aquatic organisms is not well known. Several studies showed that in the case of tritiated water, its internalization in zebrafish in its early development stages (4 days post fecundation) increases linearly with the tritium activity in the water. Several deleterious effects arising from an exposure to tritium include modified gene expression, increase in DNA damage and gamma-H2AX foci, leading to phenotypes as well changes in the swimming behavior,as and muscle alterations. This study focuses on tritiated thymidine, a form of organic tritium. , as aA study on the mussel has showed already shown that tritiated glycine induced a stronger toxicity than tritiated water. Our study’s results could also lead to a better understanding of the effects observed after an exposure to tritiated water . A focus was also made on micro- and nano-dosimetry simulations, as multiple studies showed that it leads to a better understanding the effects of the radiation repartition of the radiation, and the damages that could arise from it. In order to study tritiated thymidine the toxicity of tritiated thymidine on zebrafish in its early development stages, its internalization and cellular repartition needed to be studiedcharacterized. Zebrafish embryos (3.5 hpf) were exposed to tritiated thymidine at different activity concentrations (2.4x103 to 5.95x105 Bq/mL) and the tritium internalization was measured in zebrafish embryos, larvae and their DNA after 1 and 4 days of exposure. Microdosimetry obtained by Monte Carlo simulations was also used in order to characterize represent the cellular repartition of the energy deposits. Experimental results showed that tritium was mainly internalized in its organic form, and that the internalization increased exponentially with the external activity before reaching a saturating point at around 1200 and 600 µGy/h when embryos and larvae were incubated at around 3x105 Bq/mL for embryos and larvae, respectively (log-logistic increase). Results also showeshow d that in presence of at 3.2x103 and 3.7x104 Bq/mL, tritium levels in DNA were of at 0.2 and 1.2 Bq/µg of DNA (???) respectively. Those levels were not statistically different after 24 and 96 hours of exposure. Microdosimetry simulations showed that the mean imparted energy deposited by the tritium decay increased linearly with the cell radius. The simulation results also showed that smaller cells were more at risk of being the target of small energy deposits. Those energy deposits also being closer to each other, it means the cells are could be at a higher biological risk. Deleterious effects as development defects, swimming behavior perturbations, DNA damage inductions, or gene expression modifications after exposure to 30 and 150 µGy/h are under investigation, and DNA damage will be linked to nano-dosimetry simulations giving theoretical information about the DNA damage that could arise from our this type of exposures.