  We consider a collective behavior model in which individuals try to imitate
each others' velocity and have a preferred speed. We show that a phase change
phenomenon takes place as diffusion decreases, bringing the system from a
"disordered" to an "ordered" state. This effect is related to recently noticed
phenomena for the diffusive Vicsek model. We also carry out numerical
simulations of the system and give further details on the phase transition.
