In subjects who had undergone monopedal bicycle ergometer work for 30 min 4 days a week for 6 weeks under aerobic conditions, biopsies from the M. vastus lateralis were studied in the light and electron microscope. In some cases very small myofibers with central nuclei were found. At the ultrastructural level, these fibers turned out to be myotubes. The satellite cells occasionally appeared to be activated and to fuse with muscle fibers. It was concluded that these observations represented evidence for the development of new muscle fibers.