The influences of newcomers' entry on oldcomers in a class for 4-year-olds were investigated from the viewpoint of adaptation in transition period. Three oldcomer boys were observed for three preschool months from March to May by means of a videotape recorder and field notes. Changes of playmates, decreases in length of playtime with playmates, and changes of strategies to deal with playmates were found to occur. There was a time lag in the emergence of influences to each of the target boys. The results were interpreted to show that a large change in a new system emerged through interaction of two factors : changes in existing peer groups and influences by newcomers' entry.