In this paper, we investigate leadership related to the instruction of lower division undergraduate courses at five university mathematics departments with strong calculus programs. We use social network analysis to identify patterns of influence on instruction, using the relations: advice seeking, instructional material sharing, discussion of instructional matters, and explicit influence on teaching approach. Data were gathered through social network surveys, which received over 60% response rate at each site. Considering both formal and informal phenomena, we identify variation in vertical and distributed instructional leadership structures within these departments. In these five communities, not all those with hierarchical authority have real influence over instructional practice, but those with the most influence over instruction do hold formally recognized positions. In light of these findings, we discuss implications for future research and practice in higher education.