This study examined the bidirectional relationship between parenting and boys' externalizing behaviors in a four-wave longitudinal study of toddlers. Participants were 104 intact two-parent families with toddler sons. When their sons were 17, 23, 29, and 35 months of age, mothers and fathers reported on a broad range of parenting dimensions (support, lack of structure, positive discipline, psychological control, and physical punishment). In addition, mothers reported about their sons' externalizing behaviors. Results from structural equation modeling did not support a bidirectional model of parenting and externalizing behavior among toddler boys. Although parenting did not predict boys' externalizing behaviors, results showed that at 23, 29, and 35 months of age, boys' externalizing behavior predicted parent-reported support, lack of structure, psychological control and physical punishment. Additional analyses indicated that these child-effects were equally strong across time and across mothers and fathers. Results indicate that it is important to offer both mothers and fathers support when dealing with increases in toddlers boys' externalizing behavior and that parenting programs should not only focus on reducing harsh discipline tactics, but also on encouraging positive parenting behavior. (Contains 4 figures and 2 tables.)