This paper introduces the novel concept of ghost-FSM as a BIST structure for sequential machines. A ghost-FSM is a contrive implicit machine that is deliberately embedded within the framewor of a given FSM. A ghost-FSM remains dormant except at the testing phase, when it helps to generate those test patterns that, otherwise, coul not be generate due to certain ideosyncracies of finite state machines. It acts as an effective tool to enhance the fault coverage of a sequential machine in a PRPG base BIST environment, without affecting the normal operating mode of the machine. Extensive experimentation, carried out on MCNC benchmarks, confirm that significant improvement in testability is attained with occasional, very little, area overhead.