Using latent growth curve modeling, the current study investigated gender moderation of the longitudinal pathways from internalizing to both social competency (i.e., social assertiveness) and the initiation of substance use (i.e., tobacco, alcohol, marijuana), as well as the effect of a preventive intervention on that process. Rural Midwestern adolescents who were participating in a school-based preventive intervention study were an average of 12.3 years old at the pretest assessment conducted in 1998. A latent growth curve comparison analysis found that internalizing was related inversely to initial levels of social assertiveness skill among girls; further, internalizing was related positively to substance use initiation growth trajectories among girls. Girls who participated in the preventive intervention demonstrated a slower increase over time in substance use initiation and a faster increase in social assertiveness. Gender moderation of the impact of internalizing and social assertiveness on substance use initiation and response to the intervention, as well as the utility of latent growth curve modeling in the study of longitudinal change, are discussed.