Although the relationship between temporomandibular disorder and forward head posture (FHP) is controversial, it is generally accepted that altered head posture can affect mandible position and masticatory muscles activity. Because suprahyoid (SH) and infrahyoid (IH) muscles are stretched by increased passive tension in FHP, this study investigated their activity during mouth opening in FHP compared to neutral head posture (NHP). Twenty healthy subjects (10 males and 10 females) participated in this study. Head postures were evaluated with a cervical range of motion instrument. Electromyography (EMG) activity of bilateral SH and IH muscles was measured while an open mouth was maintained at each head posture. Paired t-test was used to identify significant differences in normalized EMG activity between head postures. Statistical significance was set at .01. Results showed the normalized EMG activity of SH and IH muscles were significantly lower in FHP compared to NHP. This finding indicates that FHP affects the EMG activity of hyoid muscles when they are stretched.