This longitudinal study aimed to investigate infants' spontaneous object interactions during naturalistic longitudinal observations in a day care centre in Japan. Infants' and caregivers' interactions during free play time were videotaped. The main focus related to how infants' object interactions changed during locomotor development. Observations showed that all nine infants gradually acquired their locomotor skills, they spent about 50% of their time in contact with objects and as infants developed, they more frequently travelled to objects and carried them and their locomotion became less destination-directed. Constantly-oriented travelling episodes became less frequent, and object-carrying behaviours without explicit destinations became more frequent. The results suggested that these changes were due to infants' posture, not their locomotor status.