Interactions between fibronectin (FN) and glycosaminoglycans are essential for extracellular matrix morphology and cell adhesion. One of the most abundant glycosaminoglycans is chondroitin sulfate, and here we show that recombinant FNs (deminectins (DN)) containing the carboxyl-terminal cell, heparin, and fibrin domains bind specifically to chondroitin sulfate in affinity chromatography assays. Using a panel of mutant DNs, important determinants for chondroitin sulfate binding have been localized to repeats III13 and III14 within the heparin domain. In particular, mutation of an arginine pair in repeat III13 to neutral residues ablated binding to chondroitin sulfate as we previously reported for heparin (Barkalow, F.J.B., and Schwarzbauer, J.E. (1991) J. Biol. Chem. 266, 7812-7818). These results, in combination with the ability of heparin and chondroitin sulfate to compete for binding to DNs, demonstrate that these two glycosaminoglycans interact with similar or overlapping sites in FN. One important difference between FN interactions with heparin and chondroitin sulfate is that, while FN and DNs bound equally to heparin, FN bound less efficiently than DNs to chondroitin sulfate. Reduced binding to chondroitin sulfate was also observed with a larger recombinant FN lacking internal repeats III1-7 indicating that the amino-terminal region acts to limit binding to the carboxyl-terminal domain. Our results demonstrate that interactions between FN and chondroitin sulfate are modulated by molecular context.