Theoretical models posit real-time bidirectional processes between parents and children as integral to child development, yet few studies have examined such processes. In this study, cross-lagged autoregressive latent growth models were used to assess the dynamic organization of mother and toddler behavior across a snack-delay task. Maternal support and nonsupport and toddler aversive behaviors were rated in 15-s intervals (N = 127 dyads; mean toddler age = 32.7 months). As hypothesized, within-mother increases in nonsupport predicted within-child increases in aversive behavior in the subsequent interval. Likewise, within-child increases in aversive behavior predicted within-mother increases in nonsupport. Maternal support was unassociated with child aversive behavior. Findings are consistent with a dynamic model of mother-toddler influences on negative behaviors during a regulatory challenge. (PsycINFO Database Record (c) 2019 APA, all rights reserved).