A control valve having a shiftable valve plug includes a valve body having an inlet, an outlet, a flow passage extending between the inlet and the outlet, and defining a longitudinal axis, a generally cylindrical valve cage disposed within the valve body and sized to receive the valve plug, with the valve cage including at least one aperture comprising at least a portion of the flow passage, a portion of the valve cage disposed adjacent a portion of the valve body at an interface. A seal having a C-shaped cross section is provided and is disposed at the interface between the valve cage and the valve body, and a retainer is disposed adjacent the interface and positioned to secure the seal against movement along the longitudinal axis.