Vital "reptational" movements of isolated synthetic polymer chains on a substrate were imaged by atomic force microscopy (AFM) in the tapping mode. The isotactic poly(methyl methacrylate) molecules deposited on mica have long flexible chains that reptated on the substrate like snakes (caterpillar-like) along their chain directions in humid air. The thin water layer adsorbed on the substrate accelerated these movements. Possible tip scan effects on the observed movements were evaluated and found to be small mainly based on the following points: (I) the directions and time scales of the movements were not related to those of the scan, (2) the movements of the chains during interruption of the scan were similar to the movements during scanning, and (3) the movements were also observed in the frequency modulation mode (namely, "noncontact" mode) in which the tip force acting on the chains is significantly smaller than that in the tapping mode. The dynamic information about single polymer chains will stimulate new insights into the understanding of polymers, especially in surface phenomena, such as adhesion, wetting, and friction, and is also essential for fabricating advanced nanomaterials based on polymers.