The pattern of electromyographic activity of the equine long back muscle at the walk has not yet been reported. The aim of this study was to use surface electromyography to measure activity of the longissimus dorsi muscles of horses walking on a treadmill. Fifteen horses without back pain were used and electromyographs were recorded bilaterally from the longissimus dorsi muscles at the level of T12, T16 and L3. Mean electromyograph activity and mean motion were calculated for each horse. At the walk, only one maximum activity for each longissimus dorsi muscle was detected during each motion cycle and this was highest at T12 and lowest at L3. Activity of the longissimus dorsi muscles at the walk is mainly responsible for stabilisation of the vertebral column against dynamic forces. At T12 the high maximum activity could contribute to the development of muscle pain at this site.