Chain elongation suppression of cyclic block copolymers in microphase-separated bulk was determined quantitatively. Solvent-cast and annealed films are confirmed to show alternating lamellar structure and their microdomain spacing D increases with increasing total molecular weight M according to the relationship Dproportional toM(0.59), which agrees quite consistently with the theoretically predicted power law, i.e., Dproportional toM(3/5). This result is in contrast to the well-established issue for linear block copolymers, where the relationship Dproportional toM(2/3) has been confirmed to hold both experimentally and theoretically. This means that chain elongation of each component block is suppressed considerably, owing to their looped conformation in strongly segregated bulk. (C) 2004 American Institute of Physics.