Prospective longitudinal data from over 14,000 youth residing in 28 communities in the rural United States were analyzed to examine the emergence of mixed-sex friendship groups in early adolescence. Youth were surveyed on 5 occasions between fall of 6th grade and spring of 9th grade. At each assessment, youth reported the names of up to 7 same-grade friends and described patterns of alcohol use, cigarette use, and delinquency. Approximately 800-900 friendship groups (M = 10.5 members) were identified at each assessment and categorized in terms of gender composition (all-girl, mostly-girl, mixed-sex, mostly-boy, all-boy). The proportion of groups categorized as mixed-sex increased with grade level (10% in 6th grade, 22% in 9th grade), but gender-homogenous groups predominated at all grade levels (76% in 6th grade, 51% in 9th grade). Mixed-sex groups were slightly larger than all-girl groups but the same size as all-boy groups. All-girl groups had the highest levels of tight-knittedness (i.e., density, reciprocity, and transitivity), with mixed-sex groups having the lowest levels and all-boy groups having intermediate levels. After controlling for demographic factors, future mixed-sex group membership was predicted by lower popularity, higher levels of delinquency, and lower levels of alcohol use; mixed-sex friendship group membership was associated with increased likelihood of cigarette use. Results are partially consistent with Dunphy's (1969) classic account of the emergence of mixed-sex groups in adolescence, but suggest that in early adolescence, mixed-sex group affiliation is significantly associated with deviant behavior and peripheral social status, not with popularity. (PsycINFO Database Record (c) 2014 APA, all rights reserved).