Using data from the Early Head Start Research and Evaluation Project, a cross-lag mediation model was tested to examine longitudinal relations among low-income mothers' sensitivity; toddlers' engagement of their mothers; and toddler's self-regulation at ages 1, 2, and 3 years (N = 2,958). Age 1 maternal sensitivity predicted self-regulation at ages 2 and 3 years, and age 2 engagement of mother mediated the relation between age 1 maternal sensitivity and age 3 self-regulation. Lagged relations from toddler self-regulation at ages 1 and 2 years to later maternal sensitivity were not significant, suggesting stronger influence from mother to toddler than vice versa. Model fit was similar regardless of child gender and depth of family poverty.