A study was made of the morphology and fibre type composition of the adductor mandibulae (AM) muscle in Teleosts with very protrusible (carp), moderately protrusible (cod) and non-protrusible (trout and cat-fish) jaws. In contrast to the trout and cat-fish, in which the AM is formed by only 2 components (mandibular and mental), in the carp and cod there is a third portion (maxillary) which is more or less developed in relation to the extent of jaw protrusion. Fibre types were identified by means of histochemical staining for succinate dehydrogenase and myosin ATPase activities, and by immunohistochemistry with anti-sera specific for fish fast and slow myosins. In all the species AM is composed principally of white (fast) fibres, with a smaller proportion of red (slow) fibres. The red fibres, which appear in the deep layers only of the muscle are not found in all of the components, and in the different species are not always present in the same parts. In those parts of the AM which are mixed, a transition zone lies between the red and white areas, and is usually composed of a third, or intermediate, type of fibre with histochemical and immunohistochemical properties similar to those of the pink zone of lateral muscle. The anatomical characteristics and different fibre type compositions of the various components forming the AM are discussed in relation to the extent of jaw protrusion and the relevant physiological data concerning other movements in which this muscle participants.