During waking h, the existence of ultradian rhythms in gross motor activity has been described in nonprimates, nonhuman primates and newborn humans, but not in adult humans. Some of the previous studies suggested that the appearance of these rhythms could be favored by conditions of isolation and low environmental demands. To confirm the existence of ultradian rhythmicity in the gross motor activity of adult humans and to describe their characteristics, an actimeter was used to record the mobility of 13 adults who remained alone and isolated for 5 h (15:00-20:00) in a monotonous environment with nothing to do. Least squares rhythmometry analysis showed that the gross motor activity of 12 out of 13 subjects had significant rhythms (p < 0.05) within the ultradian band, with periods of between 0.5 and 2.5 h. There were important individual differences between the dominant periods and the same subject might show more than one significant period. These rhythms were stable and they tended to appear immediately the experiment began. The results suggest that a philogenetically old mechanism that organizes gross motor activity in ultradian rhythms exists in adult humans.