This paper examines the tracking/rfejection problem of an adaptive controller using the internal model principle. The major contributions are to establish the tracking/rejection properties of the controller and to relax the order restriction of the filters used in some existing continuous time adaptive systems. It is shown that, in the presence unmodelled dynamics and external disturbances, the controller can give a small 'in the mean’ tracking error after the establishment of global bounded-input bounded-output stability of the adaptive system. Perfect tracking/rejection of deterministic trajectories/disturbances can be achieved when the unmodelled dynamics disappear. For the implementation of the controller, no a priori knowledge on the modelling error is required. The adaptive scheme was also tested to be effective by simulation studies.