This study examined two types of muscle fibre, associated with entirely different tasks in the cyprid of Balanus amphitrite (Crustacea: Cirripedia), and detected a striking difference. The first fibre type is found in the antennulary system, which is used for selecting the most suitable substrate for continuance of the cirripede life cycle. The second fibre type is associated with the stereotyped swimming behaviour of the cyprid. The antennulary fibres have the characteristics of ‘slow’ fibres, whereas the thoracic fibres display the properties of ‘fast’ muscle fibres. The difference between the thoracic and antennulary muscles emphasizes the specialized morphology of the cirripede cyprid linked to their unique role in settlement.