Rectus capitis posterior muscles are located in the suboccipital triangle and function in extension and lateral rotation of the head and neck. There are two of these muscles on each side: the rectus capitis posterior major and the rectus capitis posterior minor. This study describes the presence of a third suboccipital muscle in a 55-year-old cadaver. In addition to highlighting the possible relationship between these muscles and cervicogenic headaches or balance in an individual, we also relate this observation to similar musculature in the cat that have been shown to effect crucial functions. © 2005 Wiley-Liss, Inc.