Single twitch muscle fibres have been isolated from various parts of the iliofibularis muscle of Xenopus laevis. After measurements of their isotonic contractile properties, myosin extraction was performed on individual fibres and the extract analysed by various forms of gel electrophoresis. In agreement with previous results three major fibre types, types 1, 2, and 3 could be discerned. Both mechanical data and native isomyosin patterns indicated a further subdivision of types 1 and 2 into subtypes (1n, 1s and 2f, 2n, respectively). Transitional forms between 2n and type 3 were also identified. Types 1 and 2 had the same kinds of light chains (LC1f, LC2, LC3f), but different heavy chains (HC) as observed on 7% SDS gels. Types 3 lacked LC3 and had a more slowly migrating LC1 (LC1s); their HC migration velocity was indistinguishable from that of type 2 HC. A comparison was made between LC1/LC3 ratio and contractile parameters for nine type 1n fibres and six type 2n fibres. No clear correlation was observed between light chain proportions on the one hand and force per cross-sectional area or shortening velocity on the other. It is concluded that myosin heavy chain composition is the major determinant for contractile performance in Xenopus skeletal muscle fibres.