Abstract It is important to understand constructs essential to low-fat-diet and physical-activity behaviors of low-income culturally diverse middle-school-age students, because ethnicity and socioeconomic status influence body weight status, and these groups are most at risk for obesity later in life. In this descriptive study, constructs important in low-fat diet and physical activity in low- to middle-income, culturally diverse middle school students were examined. Revised for cultural/developmental appropriateness and reading level, instruments incorporating constructs from the Health Promotion and Transtheoretical Models were tested with 221 youth. Results demonstrated that percentage of fat in diet, total number of strategies used for a low-fat diet, access to low-fat food, and total number of exercise processes varied significantly (p < 0.05) across stages of change for low-fat diet. We concluded that interventions should foster access to low-fat foods and processes for diet and activity change. Despite significantly lower income and higher grade level (both of which are risks for poor health behaviors), students in the private school demonstrated significantly lower fat in diet, higher perception of benefits, and better access to low-fat food. These findings warrant additional study.