The primary objective of this study was to examine parenting self-efficacy as a potential mediator of the effects of competence promoting and inhibiting parenting behavior on toddlers' scores on the Mental Scale of the Bayley. Sixty-eight predominantly middle-class mother-toddler pairs completed self-report questionnaires and toddlers were administered the Bayley Scales of Infant Development (BSID-II). Parental behaviors likely to have a detrimental effect on toddlers' ability to perform effectively on the Mental Scale of the Bayley were associated with lower actual performance by toddlers through mediation by parenting self-efficacy. Specific behaviors incorporated into the Competence Inhibiting composite variable included forceful redirection of the child's attention, ignoring and reinforcing misbehavior, potentially distracting self-conscious behaviors (fidgeting and a shifting body posture), and a pronounced expression of displeasure, anger, and/or frustration in response to the child's task behavior. The effect of ignoring child misbehavior alone was also mediated by self-efficacy. Several additional significant associations were observed among the various indicators of parenting quality, parenting self-efficacy, and toddler scores on the Bayley. Finally, relations between parenting self-efficacy and various forms of parenting behavior differed based on child temperament. Implications of the findings are discussed.