Undergraduate researchers at research universities are often mentored by graduate students or postdoctoral researchers (referred to collectively as "postgraduates") and faculty, creating a mentoring triad structure. Triads differ based on whether the undergraduate, postgraduate, and faculty member interact with one another about the undergraduate's research. Using a social capital theory framework, we hypothesized that different triad structures provide undergraduates with varying resources (e.g., information, advice, psychosocial support) from the postgraduates and/or faculty, which would affect the undergraduates' research outcomes. To test this, we collected data from a national sample of undergraduate life science researchers about their mentoring triad structure and a range of outcomes associated with research experiences, such as perceived gains in their abilities to think and work like scientists, science identity, and intentions to enroll in a PhD program. Undergraduates mentored by postgraduates alone reported positive outcomes, indicating that postgraduates can be effective mentors. However, undergraduates who interacted directly with faculty realized greater outcomes, suggesting that faculty interaction is important for undergraduates to realize the full benefits of research. The "closed triad," in which undergraduates, postgraduates, and faculty all interact directly, appeared to be uniquely beneficial; these undergraduates reported the highest gains in thinking and working like a scientist.