The present study examined the interplay among negative emotionality, responsive parenting and socio-cognitive developmental outcomes (i.e., communication, personal-social and problem-solving outcomes) in about 1620 Korean children using three waves of longitudinal data spanning the first 2 years of their life. Results from the Structural Equation Modeling (SEM) demonstrated that there were moderate to low degrees of stability in negative emotionality, responsive parenting and socio-cognitive developmental outcomes from infancy to toddlerhood. Evidence for reciprocity in the parent-child relationship was found; responsive parenting predicted higher levels of subsequent child communication (in infancy and toddlerhood), and infants' higher problem-solving ability predicted higher responsive parenting in toddlerhood. Overall, the cross-age associations among the variables were similar between boys and girls, but some different patterns were observed: when controlling for family contextual factors and the within-time correlations, negative emotionality at an earlier point significantly predicted lower responsive parenting at a later point and vice versa only in girls during infancy, but neither in boys nor in toddlerhood. The implications of these findings are discussed.