Naturalistic, longitudinal observations of 20 normal infants biweekly during their first year showed that they performed a great quantity and variety of rhythmical and highly stereotyped behaviours. Forty-seven movement patterns are described involving the legs and feet; the head and face; the arms, hands, and fingers; and the whole torso in various postures. These behaviours showed developmental regularities as well as constancy of form and distribution. Groups of stereotypies involving particular parts of the body or postures had characteristic ages of onset, peak performance, and decline. The onset of particular stereotypy groups was highly correlated with motor development. It is proposed that rhythmical stereotypies are manifestations of incomplete cortical control of endogenous patterning in maturing neuromuscular pathways.