This study examined three potential moderators of the relations between maternal parenting stress and preschoolers' adjustment problems: a genetic polymorphismthe short allele of the serotonin transporter (5-HTTLPR, ss/sl allele) gene, a physiological indicatorchildren's baseline respiratory sinus arrhythmia (RSA), and a behavioral indicatormothers' reports of children's negative emotionality. A total of 108 mothers (M-age=30.68 years, SDage=6.06) reported on their parenting stress as well as their preschoolers' (M-age=3.50 years, SDage=0.51, 61% boys) negative emotionality and internalizing, externalizing, and sleep problems. Results indicated that the genetic sensitivity variable functioned according to a differential susceptibility model; however, the results involving physiological and behavioral sensitivity factors were most consistent with a diathesis-stress framework. Implications for prevention and intervention efforts to counter the effects of parenting stress are discussed.