Acetic/nitric reagent (90% acetic acid, 10% nitric acid) has been used in the purification and semimicro determination of cellulose (Updegraff, Anal. Biochem. 32:425, 1969). We have found that acetic/nitric reagent-treated cotton fibers yield distinct fiber segments with relatively uniform minimum segment lengths. Segments from cottons of varying length, strength, elongation, fineness and maturity were examined by a light microscope linked to an electronic image analyzer. Similarities and differences in relationships of these characteristics to segment length are reported and the possibility of repeating unit length is presented.