Purpose: To (1) assess the reliability of the newly developed Fears of Stranger Danger (FSD) scale, (2) examine measurement invariance and identify demographic variation in FSD, and (3) examine associations of FSD with physical activity, screen time, and body mass index (BMI) z score. Design: Cross-sectional survey with test-retest. Setting: Neighborhoods with various socioeconomic characteristics and walkability in San Diego, Boston, and Cincinnati. Participants: Parent-adolescent pairs (n = 171), and parents of children (n = 116). Response rate was 47% for Survey 1, and 69% were retained for Survey 2. Methods: Data analyses included test-retest reliability and internal consistency for FSD, tests of differential functioning for measurement invariance, t-test for associations between FSD and demographic variables, and partial correlation for associations of FSD with physical activity, screen time, and BMI z score. Results: The FSD scale had moderate to substantial test-retest reliability (intraclass correlation coefficient =.65-.85) and excellent internal consistency (Cronbach á =.88-.94). Measurement invariance was established across gender, race/ethnicity, and income. FSD was higher regarding younger children, females, nonwhites, and lower-income youth. FSD was positively associated with restrictive parental rules for playing outside (partial r =.28-.33), and negatively associated with children's outdoor physical activity in the neighborhood (partial r = 2.27), but not associated with other measures of physical activity, screen time, or BMI z score. Conclusion: The new measure of FSD had good evidence of reliability and measurement invariance, but there were inconsistent associations of FSD with youth physical activity. Copyright © 2012 by American Journal of Health Promotion, Inc.