The development of posture in the prone and supine positions was studied longitudinally in 10 low risk preterm infants between 31 and 39 weeks' postmenstrual age whose subsequent neurological development at 18 months old was normal. The infants were observed from one week after birth until the expected date of delivery. All postures were analysed according to the procedures described by Prechtl et al. There were large intraindividual and interindividual differences in posture in terms of the duration and incidence of the two positions. There was no age specific preferred posture found in either position. The posture duration in the prone position was longer than that in the supine position, but overall flexed and abducted posture were observed more often in the former. Turning of the head to the right side was also observed more often in the prone than in the supine position.