This study examined the relationships between social network position and the use of tobacco, alcohol, marijuana, and inhalants in a sample of 1,119 sixth-grade youth. Social network analyses of peer nominations were used to categorize youth as "members" of social groups, "liaisons" between groups, or social "isolates." The results revealed that liaisons were more likely to use tobacco than members or isolates and were more likely to use alcohol than isolates. Youth in the three social positions did not differ on their use of marijuana or inhalants. The findings are discussed with reference to previous social network research on adolescent relationships and substance use. The effects of gender, ethnicity, and residence location on substance use also are reported. (Contains 1 figure and 2 tables.)