Abstract: The flow between counter-rotating disks for aspect ratio R/h = 7, is investigated numerically. This flow gives rise to a new instability pattern, a circular chain of vortices connected to the boundary layers trough spiral arms. The physical mechanism for this instability is elucidated: the balance between the centrifugal effects of each rotating disk results in the detachment of the boundary layer over the slower rotating disk, leading to a free shear layer in the bulk of the flow, witch becomes unstable via a Kelvin-helmholtz-type instability.