Nonshared environmental influences have been found to be important for adolescent development. This study of 516 families investigated whether differential parental negativity or warmth is linked to adolescent adjustment apart from the effect of the level of parenting toward each child separately. After accounting for level of parental treatment to the adolescent, the authors found that differential parenting to the siblings contributed unique variance in adjustment. Significant interactions were found between level of parenting and differential parenting. In each case, differential parenting was more strongly linked to adjustment when the level of parenting was low in warmth or high in negativity. These results are indirect evidence that differential parenting can be considered a within-family influence on sibling adjustment and as direct evidence that nonshared environmental factors may systematically vary in strength between families.