1. Five major fibre types in chicken skeletal muscles are recognized, based upon their histochemical and morphological characteristics. A classification of these which is readily related to a commonly used classification of mammalian muscle fibre types is given.2. Seven muscles of the chicken were analysed in recognizing this range of fibre types. The proportions of the different types in each of these were determined. In some cases a gradient of fibre type composition exists across a single muscle.3. Measurements of muscle contraction were used in defining tonic muscles, which contain two fibre types. It was shown that in addition to the anterior latissimus dorsi (a.l.d.), previously well known to be a tonic muscle, two other muscles, the plantaris and the adductor profundus, are of the same class, but differ subtly from the a.l.d. in certain features. Gross red colouration is not a useful diagnostic feature of slow muscles, since the tonic adductor profundus, for example, is white.4. Fibres similar histochemically to mammalian type I (slow-twitch) occur in some of the avian twitch muscles investigated. These are oxidative in character, and despite the fact that they are multiply innervated we suggest that these are avian slow-twitch fibres.5. The patterns of cholinesterases found in a skeletal muscle correspond to its fibre type composition, with regard to both the concentrations and the proportions of the multiple forms of enzyme present. The distinctive patterns of those forms of acetylcholinesterase in the different fibre types are described.6. The fibre type composition is changed by inherited muscular dystrophy in a characteristic manner. This change has so far been found (at the earlier stages of the disease) only in the muscles with a predominance of type II B fibres in the normal chicken. Pathological changes within the fibres occur selectively in the type II B fibres, but there are exceptions to this and the effect can be greatly modified by the type of neighbouring fibres.