A study was undertaken of the settlement and attachment of larvae of Lepeophtheirus salmonis (Krøyer, 1837) on a salmonid host Salmo salar Linnaeus, 1758. Preferred settlement sites were the fins and other protected areas. Initial, reversible settlement was followed by attachment with the hooked second antennae. The filament used for subsequent attachment was found to comprise three distinct regions; basal plate, stem and external lamina. The filament was secured to the epithelial basement membrane by the basal plate which was formed by secretion down the axial duct from the posterior A-group of cells. The stem was fibrous and carried the axial duct along its length. A thin external lamina covered the stem and was apparently continuous with the chalimus cuticle. A number of filament-associated organs are also described, comprising cell groups A-C although only the function of A was characterized.