The aim of the present research was to design a groupal program of intervention for adolescents and assess its effects on behavioral and cognitive variables of social interaction. This study used a pretest-intervention-posttest design with control groups. The sample consisted of 174 subjects aged 12 to 14. There were 125 experimental subjects and 49 control subjects. Before and after the program, five instruments were administered to measure: assertiveness, cognitive strategies of social interaction, and social behaviors such as prosocial behaviors, leadership, selfcontrol, anxiety-shyness. The intervention program applied to the experimental subjects consisted of a two-hour intervention session per week throughout one academic year. All of the 60 activities stimulate communication, friendly cooperative interactions, expression and understanding of emotions, identification of perceptions and prejudices as well as solving of human conflicts. Results of MANCOVAs and ANCOVAs suggest that there was a very positive effect (p < 0.05) of the program. There was an increase of self-assertiveness, assertive social behaviors, leadership behaviors, cognitive assertive strategies of social interaction and a decrease of anxiety-shyness behaviors. The data suggest that the experience of the intervention was particularly positive for those adolescents who, beforehand, had had a low social development.