The geniohyoid has been characterized as a parallel-fibred muscle extending from the mandibular symphysis to the hyoid body. Motor end-plate staining of dissected whole muscles showed that only in the hamster does the geniohyoid architecture correspond to the usual description of simple parallel fibres. In rats and mice, the muscle was complex with two banks of myofibres separated by a tendinous intersection. Geniohyoid was attached mainly to a basihyal raphe shared by the sternohyoid. A septate geniohyoid may be a primitive feature of mammals because it also exists in the opossum and tree shrew. The segmental disposition of the geniohyoid in rats and mice reflects its serial homology with other compound ventral muscles, such as sternohyoid and rectus abdominis. Architectural patterns of myofibres and connective tissue were distinctive for each species.