Biocompatibility restrictions have limited the use of magnetic nanoparticles for magnetic hyperthermia therapy to iron oxides, namely magnetite (Fe3O4) and maghemite (gamma-Fe2O3). However, there is yet another magnetic iron oxide phase that has not been considered so far, in spite of its unique magnetic properties: epsilon-Fe2O3. Indeed, whereas Fe(3)O(4)and gamma-Fe(2)O(3)have a relatively low magnetic coercivity, epsilon-Fe(2)O(3)exhibits a giant coercivity. In this report, the heating power of epsilon-Fe(2)O(3)nanoparticles in comparison with gamma-Fe(2)O(3)nanoparticles of similar size (similar to 20 nm) was measured in a wide range of field frequencies and amplitudes, in uncoated and polymer-coated samples. It was found that epsilon-Fe(2)O(3)nanoparticles primarily heat in the low-frequency regime (20-100 kHz) in media whose viscosity is similar to that of cell cytoplasm. In contrast, gamma-Fe(2)O(3)nanoparticles heat more effectively in the high frequency range (400-900 kHz). Cell culture experiments exhibited no toxicity in a wide range of nanoparticle concentrations and a high internalization rate. In conclusion, the performance of epsilon-Fe(2)O(3)nanoparticles is slightly inferior to that of gamma-Fe(2)O(3)nanoparticles in human magnetic hyperthermia applications. However, these epsilon-Fe(2)O(3)nanoparticles open the way for switchable magnetic heating owing to their distinct response to frequency.