This study examined the mediating role of emotional availability in the relation between parent positive personality and toddler problem behavior. The sample comprised 654 twins at 12 and 32 months. Primary caregivers (>95% mothers) completed six measures indexing parent positive personality assessed at 12 months, and emotional availability and toddler problem behaviors assessed at both times. Parent positive personality was positively associated with emotional availability and negatively associated with problem behavior. The indirect path from parent positive personality to problem behavior through emotional availability was significant, and parent positive personality was no longer significantly related to problem behavior, suggesting full mediation. Although problem behaviors were heritable, over half of the variance was due to the shared environment, pointing to early childhood as a time when parent characteristics and behaviors may have maximum influence. Parental emotional responsiveness could serve to reduce problem behaviors in young children.