The occurrence and chemical nature of the cross-links between cellulose microfibrils in outer epidermal cell walls in Pisum sativum cv. Alaska was investigated by rapid-freezing and deep-etching techniques coupled with chemical and enzymatic treatments. The cell wall in the elongating region of epidermal cells was characterized by the absence of the cross-links, while in the elongated region, the cell wall was characterized by the presence of cross-links. The cross-links remained in the cell wall of the elongated region after treatment with SDS electrophoresis sample buffer and treatment with 4% potassium hydroxide. After treatment with endo-1,4-beta-glucanase, which fragments xyloglucan, the cross-links were remarkably reduced from the cell wall of the elongated region. The endoglucanase treatment also reduced immunogold labeling of xyloglucan in the cell wall. The endoglucanase hydrolysate from the cell wall fraction of the elongated region gave spots of oligosaccharides in thin layer chromatography, which were identical to the spots of xyloglucan oligosaccharides produced by xyloglucanase from both the cell wall fraction and tamarind xyloglucan. These results indicate that the cross-links are made of xyloglucan. We discussed the possibility of cross-links involved in the control of mechanical properties of the cell wall.