The invention relates to a mobile communication system including mobile stations, a mobile switching center, and a subscriber register containing subscriber data, which includes location information indicating the location of mobile stations. In order to detect the presence of a fraudulent mobile station the system includes a monitor for monitoring the processes which trigger location information changes in the subscriber register, and a detector for detecting and indicating the presence of a fraudulent mobile station if a location information change has been triggered by a process which should not trigger a change of location information in the subscriber register.