Highly crystalline cellulose IIII samples were prepared by subjecting oriented films consisting of an assembly of Cladophora cellulose microcrystals to supercritical ammonia. Diffraction data recorded on these specimens indicated that the crystals of cellulose IIII could be fully described with a one-chain unit cell and a P21 space group, with the cellulose chain axis on one of the 21 screw axes of the cells. The new cell had the following parameters: a = 0.448 nm, b = 0.785 nm, c (chain axis) = 1.031 nm, γ = 105.1°. In this cell, which is half of the one proposed so far for cellulose IIII, one glucosyl residue becomes the asymmetric unit. A good agreement between the diffraction analysis and spectroscopic data was observed. 13C CP/MAS spectra of the samples presented only six sharp resonance peaks, attributed to the six carbons of the asymmetric glucosyl residue. In these spectra, the occurrence of the C6 signal at 62.3 ppm is a clear indication that the hydroxymethyl moiety is in the gt conformation. F...