Many mammalian muscles have a complex internal architecture. This type of structure could allow a single muscle to produce a variety of force vectors through selective regional contractions. This hypothesis was tested electromyographically in the multipinnate pig masseter by recording simultaneously from several intramuscular sites. It was found that the activity in different portions of the masseter varied systematically during the various phases of mastication. Anatomical correlates of the differential activity included fasciculus orientation and length, sarcomere length in specific jaw positions, and histochemical fiber type. The usual assumptions made about muscles for biomechanical analysis, such as uniform contraction and constant line of action, are inappropriate for complex muscles such as the pig masseter.