The problem of controlling a robot manipulator while the end effector is in contact with all environment of finite but unknown stiffness is considered in this paper. An exponentially stal,le control law is derived starting front a passivity-based position control algorithm. The original position trajectory is scaled along the interaction direction so as to achieve force tracking as well as position tracking: along: the unconstrained directions. A passivity-based adaptive algorithm is designed to avoid the explicit computation of the scaling factor, which depends on the unknown stiffness of the environment, leading to time-varying PID control actions on the force error.