Magnetic hyperthermia is a promising new treatment, allowing to locally induce a temperature increase in cancer tumors that leads to a lethal effect. For this, magnetic nanoparticles are introduced in tumors and exposed to an alternative magnetic field which produces the wanted temperature rise. While the final biological effect can be assessed by many techniques, the in situ temperature changes are often difficult to evaluate otherwise than with a regular thermometer. This fairly crude procedure does not allow to finely report changes at the tissue or cell level. In this context, we report here an original method based on a chemical nanoprobe designed to follow temperatures changes during hyperthermia therapy. In our work, AMB-1 magnetotactic bacteria produce the magnetic nanoparticles (magnetosomes), since we have already shown that this type of nanoparticles had a much better magnetic activity than chemically synthesized particles (Alphandery et al. ACS Nano, 2011, 5:6279). Interestingly, by introducing rhodamine B in an optimized growth medium for these bacteria, we were able to extract fluorescent magnetosomes with new characteristics. Indeed, keeping their typical magnetic activity useful for cancer therapy, they would also display a temperature-dependence fluorescence allowing to perform local measurements at a microscopic level in biological tissues. The molecular mechanism would be discussed, as well as results obtained with different cell types (RG2, TC1-GFP, C57NL/6 peritoneal macrophages, U87-MG) and tissues (RG2-implanted rat brain).