Electron energy loss spectroscopy (EELS) was used to study the distribution of cadmium within the microalga Tetraselmis suecica when submitted to cadmium contamination. This analytical technique, which is associated to transmission electron microscopes, demonstrated that cadmium was stored specifically in the osmiophilic vesicles of T. suecica. The EELS study of the oxidation states revealed that cadmium was stored as Cd2+. In addition, the EELS quantification showed a significant relationship between cadmium, nitrogen, and sulfur concentrations. The toxic element is probably bounded to organic molecules via S-Cd bounds.