Deviations from expected perfect symmetry can occur, and organisms can develop several kinds of asymmetry. Among others are fluctuating asymmetry (FA) and directional asymmetry (DA). FA represents small random differences between corresponding parts on the left and right sides of an individual in bilaterally paired structures; it is thought to reflect an organism"s ability to cope with genetic and environmental stress during growth. DA occurs whenever one side on the plane of symmetry develops more than the other side, and has a genetic component. In this research, we tested the presence of dental FA and DA by examining the expression of asymmetry in the upper first molar of 25 individuals belonging to the"Cavall Pirinenc Català" horse breed . Size was not different between sides, but there was significant FA. We suggest that the predominant source of detected occlusal variation is only behavioural, being a chewing-side preference, a preference for one side of the dentition during chewing, as has been described in humans.