Electron micrograph studies have been carried out on several body systems of the copepod, Calanus finmarchicus. The mid-gut epithelium possesses a layer of densely packed microvilli; there are two well-developed muscle layers, an inner strongly folded layer and an outer thinner layer, both of smooth fibres. The body musculature shows the striation typical of skeletal muscle with clear 'a' and 'i' bands, the latter with a 'z' line. Microtubules run longitudinally through the muscle fibres. The cuticle is divided into an inner zone of nine and an outer zone of seven layers; each is composed of fine lamellae and exhibits a wave-like pattern. A seta has a cuticular wall structure which is essentially similar; the setules are composed of cuticular layers only and lack protoplasmic extensions. The attachment of body muscle to cuticle is by a specialized area analogous to a tendon. Fine tubules from the plasma membrane of the 'tendon' become grouped into dense bundles of fibres which are inserted on the cuticle as tono-filaments. The nerve fibres exhibit great variation in diameter and in thickness of the myelin sheath. The developing ovum has a large nucleus, the boundary layer of which shows prominent nuclear pores. The nucleolus is very large. The male germinal epithelium also shows a particularly large ratio of nucleus to cytoplasm. The spermatophore sac wall is intensely secretory, producing droplets which appear to form a thick laminated spermatophore wall. It is suggested that secreted materials of two types in the core substance of the spermatophore arise from the vas deferens. The same materials are present in relatively small amounts in earlier stages of spermatophore development when the sperms are tightly packed together and the spermatophore wall is very thin.