The integument of Euperipatoides leukarti (Onychophora) has been studied by SEM and TEM-histochemical techniques. The thin (2-3 mu) cuticle is essentially arthropodan in design, comprising a thin, lamellate outer epicuticle, a homogeneous inner epicuticle, and an extensive fibrous procuticle. Five tanned lipoprotein lamellae constitute the outer epicuticle, the innermost showing a cross-striated pattern and the outermost also containing carbohydrates. The inner epicuticle contains untanned lipoproteins. The chitin-protein composite procuticle reveals a fibrous ultrastructure but no trace of helicoids. Distally it may be tanned, producing a sclerotin layer (exocuticle), pronounced in the sensory setae, claws and mandibles. Penetrating the epicuticle and extending into the procuticle are numerous, minute pore canals through which mobilised procuticle is resorbed prior to ecdysis. Beneath the procuticle is a monolayered epidermis traversed by tonofibrils and containing dense pigment granules. The membrane junctions are strongly convoluted and include an apical zonula adherens, septate desmosomes and a proximal lymph space. The epidermis is underlain by a thick (5-25mu) basement membrane of helicoidally organised collagen fibres. Functional and phylogetic implications of the integumental structure are discussed. Arthropod affinities are clear and the Onychophora should be included in that phylum if it is to be retained in modern taxonomic nomenclature.