A phenomenological model of the mechanism of stabilization of the body orientation during locomotion (dorsal side up) in the lamprey is presented. The mathematical modeling is based on experimental results obtained during investigations of postural control in lampreys using a combined in vivo and robotics approach. The dynamics of the model agree qualitatively with the experimental data. It is shown by computer simulations that postural correction commands from reticulospinal neurons provide information sufficient to stabilize body orientation in the lamprey. The model is based on differences between the effects exerted by the vestibular apparatus on the left and the right side.