Introduction - Using data collected as part of the triennial survey in 2003–2004, our objective was to describe dietary, physical activity, and sedentary behaviors among eighth-grade adolescents in France, and to identify the associated socioeconomic factors. Methods - Data were collected via face-to-face questionnaires administered to 7,538 adolescents. The frequencies of behaviors (such as eating breakfast and using the school cafeteria), consumption of certain food groups (fruits, vegetables, dairy products, sugary drinks), physical activity, and sedentary behavior were described. Socioeconomic factors associated with behaviors that could be considered unfavorable were identified using logistic regression. Results - Nearly two-thirds (63.7%) of adolescents reported not consuming fruit daily, 62.9% reported not consuming vegetables, 19.8% reported not consuming dairy products, and 30.9% reported not eating breakfast. More than half ate lunch in the school cafeteria every day. Furthermore, 37.3% of them never participated in sports outside of school, and 41.0% spent more than 3 hours a day in front of a screen. The adolescents’ gender, age, and family and social living conditions were factors associated with these behaviors: dietary habits, physical activity levels, and sedentary behaviors considered unhealthy were thus generally associated with unfavorable socioeconomic characteristics. Conclusion - Adolescents in the ninth grade constitute a population at risk for unhealthy dietary behaviors. These data confirm the importance of health promotion initiatives for this age group and highlight the value of considering socioeconomic factors in their development. (R.A.)