Abstract : Some multiferroic materials display simultaneous ferroelectric and magnetic order. In some of these compounds, both order parameters are coupled and applying a magnetic field can reorientate the electric polarization. This interaction is called "magnetoelectric coupling" and makes these compounds very interesting not only at a fundamental level but also for their potential application. We have synthesized single crystals of BiFeO3 and Bi0,45Dy0,55FeO3 by a flux grow method. We present a detailed study of their physical properties and show that the two compounds are ferroelectric at room-temperature. The high resistivity of the BiFeO3 single crystals allows us to measure a large spontaneous polarization of 100 μC/cm², as theoretically predicted. As the observation of a linear magnetoelectric effect is forbidden in this compound due to the presence of the cycloid, we have chosen to substitute Bi by Dy to also study the Bi0,45Dy0,55FeO3 compound. The first results on this compound are presented by comparison with magnetic and electric properties of BiFeO3. Finally, the magnetoelectric study at room temperature by neutron diffraction on a single crystal of BiFeO3 is presented. We show that the application of electric fields leads to a deep transformation of ferroelectric, ferroelastic and antiferromagnetic domains. In particular, we provide room-temperature evidence for the coupling between magnetization and electric polarization in the BiFeO3 multiferroic compound.