Advances made in spinning technology require increasingly precise controls of cotton fiber quality (DEUSSEN, 1992). By measuring parameters on a one-fiber basis, AFIS determines the variability of their physical characteristics. Field studies for quality determination (BRADOW et al., 1995; CLOUVEL et al., 1997) use small mass specimens to provide boll-by-boll analyses. Precision different from that obtained for commercial cottons may be expected in that case. The precision study described here aims to help spinners and researchers determine the number of specimens and the number of fibers per specimen required to obtain the precision they need for length, fineness and maturity measurements