A superconducting single electron transistor has been investigated in a regime where its behavior is governed by the interplay of charging and Josephson effects with the corresponding energies Ec and EJ, respectively. Using a SQUID geometry, we were able to change the ratio of EJ/Ec (from EJ/Ec≈3.5 to EJ/Ec≈0.44). In zero magnetic field, we observed a sharp Coulomb blockade at low current followed by a negative slope of the I–V curve at higher current. Applying an external magnetic field perpendicular to the substrate and thus decreasing EJ/Ec we observed an increase of the blockade voltage from 10 μV up to 45 μV and a decrease of the crossover current from 5 nA down to 0.5 nA. Such a behavior of the transistor can be explained using a Bloch band picture.