An electromyogram (EMG) is an electric signal generated when a muscle is activated. EMG signals can be used as input signals to control a myoelectric arm, a power-assist robot, and so on because EMG signals are generated before a motion. Although many kinds of control methods using EMG signals for a myoelectric arm or a power-assist robot have been proposed, the comparison between the methods is difficult because it is different what each method calculates from a measured signal, and it is not easy to define the best method. In this paper, a myoelectric arm is controlled based on EMG signals as an example of a system in which EMG signals are used as input signals. Genetic programming (GP) is used in order to construct an algorithm for a control method of a myoelectric arm.