This study of the locomotion symmetry was undertaken to provide standard symmetry indices of a group of sound horses at the trot. Using a 3D data collection system, the kinematics of the limb joints of 13 clinically nonlame horses were recorded while trotting in the standard conditions of the clinical lameness examination. A kinematic symmetry indice based on an inter-correlation method was defined and applied to the vertical displacement-time and joint angle-time diagrams of the left and right joints of the horses. For each horse, the mean symmetry indice of each joint was calculated using values from 5 trials. For each joint, these means were then averaged. To evaluate the repeatability of the locomotion symmetry, the intra- and inter-individual variabilities of the symmetry indices were studied. The levels of symmetry of the markers of the trunk were generally lower than those of the limbs. Moreover, during the 5 trials these levels of symmetry were strongly variable but their mean values were very similar from one horse to another. In our experimental conditions the trunk presented a higher degree of freedom than the limbs. This high intra-individual variability indicated also that several trials are necessary to quantify the locomotion symmetry of a trotting horse. In the same way, a lower level of symmetry of the hindlimbs, compared to the forelimbs, was proved by their lower values of symmetry indices. As opposed to the supporting role of the forelimbs, the propulsive role of the hindlimbs may explain this feature.