Trajectories of children's externalizing behavior were examined using multilevel growth curve modeling of data from the NICHD Study of Early Child Care and Youth Development. According to ratings by both mothers and caregivers/teachers when children were 2, 3, 4, 7, and 9 years old, externalizing behavior declined with age. However, mothers rated children as higher in externalizing behavior than did caregivers and teachers. Higher levels of age 9 externalizing behavior were predicted by the following factors: child male gender (for caregiver/teacher reports only), infant difficult temperament (for children with harsh mothers only), harsher maternal attitude toward discipline, higher level of maternal depression (for maternal reports only), and lower level of maternal sensitivity (especially for boys). Caregivers and teachers reported higher levels of externalizing behavior in African American children than in European American children, increasingly so over time; mothers' ratings revealed the reverse. The declining slope of externalizing behavior was predicted by infant difficult temperament for mother reports only. Additional analyses suggested that the association between parenting and externalizing behavior was bidirectional. (Contains 4 tables, 5 figures, and 1 footnote.)