The dermis of the sea cucumber Cucumaria frondosa is a mutable collagenous tissue composed of collagen fibrils, microfibrils, proteoglycans, and other soluble and insoluble components. A major constituent of the dermis is a network of 10-14 nm microfibrils which surrounds and penetrates bundles of collagen fibrils. These microfibrils, which are morphologically very similar to the fibrillin microfibrils of vertebrates, were found to be insoluble in protein denaturants, including chaotropic agents and ionic and nonionic detergents, regardless of the reduction of disulfide bonds. The microfibrils are covalently crosslinked by epsilon-(gamma-glutamyl)lysine at a concentration of 3.725 nmol/mg dry weight of purified insoluble material. The network is susceptible to proteolysis by trypsin, chymotrypsin, and pancreatic elastase, but not by bacterial collagenase. Amino acid compositional analysis of the network shows it to be composed of 25% ASX and GLX residues. Comparison with the proteins in the SwissProt database gives the network protein a high probability of being related to the mammalian protein fibrillin. The network is glycosylated: approximately 7% of the mass is constituted by neutral and amino sugars. The intact microfibrillar network cross-reacted with a well-characterized antiserum to mammalian fibrillin.