The X-ray crystallographic structure of cellulose IIIII is characterized by disorder; the unit cell (space group P2(1); a = 4.45 angstrom, b = 7.64 angstrom, c = 10.36 angstrom, alpha = beta = 90 degrees, gamma = 106.96 degrees) is occupied by one chain that is the average of statistically disordered antiparallel chains. C-13 CP/MAS NMR studies reveal the presence of three distinct molecular conformations that can be interpreted as a mixture of two different crystal forms, one equivalent to cellulose IIIII and another with two independent glucosyl conformations in the asymmetric unit. Both X-ray crystallographic and C-13 NMR spectroscopic results are consistent with an aggregated microdomain structure for cellulose IIIII. This structure can be generated from a new crystal form (space group P2(1); a = 4.45 angstrom, b = 14.64 angstrom, c = 10.36 angstrom, alpha = beta = 90 degrees, gamma = 90.05 degrees; two crystallographically independent and antiparallel chains; gt hydroxymethyl groups) by multiple dislocation defects. These defects produce microdomains of the new crystal form and cellulose IIIt that scanning microprobe diffraction studies show are distributed consistently through the cellulose IIIII fiber.