This paper addresses the position regulation problem of robot manipulators under control input constraints. It is proven that the robot system under a saturated linear PID control is semiglobally asymptotically stable, if the torque bounds are larger than gravitational torques, and if the proportional gain is large enough and the integral gain is small enough. The stability analysis makes use of singular perturbation tools and the results are illustrated via numerical simulations. (C) 2003 Published by Elsevier Science Ltd.