Granulation of juice sacs in pummelo (Citrus grandis L.) fruit was found to occur upon ripening. The juice sac tissue consisted of epidermal and subepidermal cell layers, an elongated cell layer, and juice cells. The granulation was accompanied by the appearance of opaque white regions inside transparent tissue. The subepidermal cells of the granulated tissue were disordered and the cell walls of the elongated and the juice cells were distinctly thickened. An ultrastructural study has shown that the subepidermal cell walls of granulated juice sacs were distorted and had swollen regions. The cell walls of both the elongated and the juice cells had secondary thickening with pits. The total dry weight, cellulose (as glucose), lignin, and hemicellulose (as xylose) were significantly higher and insoluble proteins were lower in granulated juice sacs than in non-granulated ones. The content of insoluble neutral sugars such as rhamnose, arabinose, mannose, and galactose decreased as a result of granulation, as did that of soluble sugars glucose, fructose, sucrose, and organic acids. Thus, it seems that granulation of pummelo fruit juice sacs is a result of lignification of the juice cells, which leads to the formation of sclerenchyma.