We study the transport of ultra cold atoms in a tight optical lattice. For identical fermions the system is insulating under an external force while for bosonic atoms it is conducting. This reflects the di erent collisional properties of the particles and reveals the role of inter-particle collisions in establishing a macroscopic transport in a perfectly periodic potential. The insulating behavior of the system is connected to the nature of the periodic potential. In fact the atoms in the lattice perform Bloch oscillations if an external force is applied. In case of identical fermions, in absence of any interactions between the particles, these oscillations can last many periods as observed experimentally. From the determination of the Bloch period we can directly measure the external force acting on the atoms realizing the first atomic interferometer with degenerate fermions.