A cDNA from Riftia pachyptila was cloned. It encodes a novel 21.3-kDa protein from the worm protective tube, named RCBP (for Riftia chitin-binding protein). On the basis of partial tube-peptide sequences previously obtained, experiments using reverse transcriptase-mediated polymerase chain reaction and rapid amplification of cDNA ends led to the complete cDNA sequence. Analysis of its deduced amino acid sequence shows the presence of two chitin-binding domains. These domains are closely related to type 2 chitin-binding domains that are restricted to the animal kingdom. We showed by affinity assay and immunogold labeling that RCBP is the first protein so far known that binds specifically beta-chitin and that is unable to bind the most common alpha-form found in chitin secreting animals. The RCBP mRNA was found to be present in specific epidermal cells from the worm body wall, but never in the chitin-secreting gland cells. This unexpected result clearly indicates that this tube protein is synthesized in specialized areas of the outer epithelium and that at least two different tissues are involved in this exoskeleton synthesis.