Data on the development of locomotion during the first two years of life were collected for 128 preterm infants (21 with varying degrees of cerebral palsy) and for 111 healthy term infants. Most stages of locomotion occurred at slightly later ages among the neurologically unimpaired preterm infants than among those born at term (age corrected for prematurity). There was no difference between preterm and term infants with regard to the number and types of pathways of locomotion, or to age at onset and type of first movements through space. By 9.5 months of age, 95 per cent of term infants and 92 per cent of preterm infants showed some ability to move through space. The infants with severe cerebral palsy had considerable delay in the development of locomotion, but this occurred to only a minor extent when the degree of cerebral palsy was mild or moderate. Most stages of locomotion occurred at slightly earlier ages for boys than for girls, but these differences were not significant at any age. The number and types of pathways of locomotion, and types of first movements, were comparable in both sexes.