Decorin is an extracellular matrix dermatan sulfate/chondroitin sulfate proteoglycan found in a variety of vertebrate species. In the extracellular matrix of mammals, decorin interacts with fibrillar collagen and regulates its morphology. We report here the occurrence and distribution of collagen type I and the peptide, CEASGIGPEVPDDRD, which is present in the human decorin proteoglycan, in the extracellular matrix of different tissues of the primitive invertebrate chordate Styela plicata. The content of collagen was estimated by hydroxyproline, and its distribution in the tissues by histochemistry. Collagen was detected biochemically in intestine, heart, pharynx and mantle, occurring in higher amounts in the heart, followed by pharynx, mantle and intestine. Histochemical analysis with Sirius red indicates that collagen is present in the extracellular matrix of intestine and pharynx. Further ultrastructural immuno-gold assays using polyclonal antibodies raised against the decorin-specific peptide CEASGIGPEVPDDRD and collagen type I showed a co-localization of these molecules. These data suggest the occurrence of a protein containing a decorin-like peptide sequence, which may be interacting with fibrillar collagen in this primitive chordate.