This study investigated the effect of full-time nonparental care during infancy on children's acquisition of developmentally inappropriate patterns of interaction with peers and teachers in preschool classrooms. Subjects, 105 children with a mean age of 53 months, were divided into 3 groups of children who had: (1) full-time nonparental care beginning prior to 9 months of age; (2) part-time nonparental care prior to 12 months and full-time nonparental care thereafter; and (3) no full-time nonparental care during the first 3 years of life. Preschool head teachers, blind to children's infancy care experiences, rated children on the Preschool Behavior Questionnaire (PBQ) and the Preschool Behavior Rating (PBR) instrument. Preschoolers were videotaped during indoor free play in their child care centers, and their social and emotional behaviors were analyzed. Mothers completed the Waters' Attachment Q-Sort ratings of their children's current attachment and dependency. Results indicated that preschoolers who had been in full-time nonparental care from prior to 9 months were rated on PBR items as more competent, and were observed as being more hostile and aggressive with peers and more noncompliant with teachers, than children who had never had full-time nonparental care. In these middle-class, mostly two-parent families, the effects of fulltime nonparental care, even if begun in the first year of life, show very small effects on child aggression once parental relationships and teacher competency are partialled out. (MM)