Although the trait of Agreeableness is broadly considered a key facet of adjustment, mental health, and socioemotional competence, surprisingly little is known about its developmental origins. Laursen and Richmond (2014) proposed that children's early difficulty poses a challenge for their future social relationships, ultimately leading to low Agreeableness. Drawing from that model, we examined a path to Agreeableness in adolescence, originating in children's early temperamental difficulty and involving bidirectional effects of parenting and children's self-regulation. In a community sample of 102 mothers, fathers, and children, we assessed children's difficulty at age 3, and parental power-assertive discipline and children's self-regulation at ages 4.5 and 5.5, using behavioral observations in lengthy interactive contexts and in standard laboratory paradigms. Agreeableness at age 14 was modeled as a latent construct, derived from mothers', fathers', and teachers' ratings. Model-fitting analyses, testing the unfolding developmental path from child difficulty to Agreeableness while controlling for continuity of parental power assertion and child self-regulation, supported a process linking early difficulty with Agreeableness at age 14 through transactions over time between the child's self-regulation and power-assertive parenting. The findings highlight the early dynamics of children's temperament characteristics and parenting in the origins of Agreeableness.