Certain observable parenting behaviors contribute to the risk of children developing internalizing and externalizing problems. Yet parenting behaviors do not affect all children uniformly and effects may depend on identifiable child characteristics. One factor is a child's biological sensitivity to the caregiving environment, an indicator of which is a stress hormone, cortisol. This longitudinal study examines two dimensions of observable parenting behaviors, responsive and rejecting/harsh. These parenting behaviors and child cortisol awakening response (CAR) were measured during home visits in a sample of 100 mostly low-income White and Latina/Hispanic mothers and their children at ages 4-6. Children's internalizing and externalizing behaviors were assessed one year later. We tested the effects of responsive and harsh/rejecting parenting on child internalizing and externalizing and examined child CAR as a moderator. Results indicated that responsive parenting predicted better child mental health as indexed by fewer internalizing and externalizing behaviors, whereas harsh/rejecting parenting predicted more internalizing behaviors. Harsh/rejecting parenting interacted with child CAR such that harsh/rejecting parenting predicted more externalizing only among children with low CAR; there was no interaction of responsive parenting with child CAR. These results elucidate how child CAR may shape mental health outcomes associated with harsh/rejecting parenting.