Many science training programs are successful at supporting students in completing their degree programs. However, it is not clear which aspects of these programs meaningfully contribute toward achieving this goal. The current longitudinal study examined a well-established science training program, the Biology Scholars Program (BSP) at the University of California, Berkeley, to see whether social connections formed in BSP and/or enthusiasm about the BSP activities are key components in contributing to students' greater integration into their professional communities at 12 months and intentions to persist at 18 months into the program. Results indicated social connections and program enthusiasm at 6 months were unassociated with science efficacy, identity, and community values. However, social connections and program enthusiasm at 12 months were generally associated with higher levels of all these variables, with science identity and community values uniquely related to greater integration. Together, results show that students' connection to faculty, staff, and peers and enthusiasm for the program activities are both key components of successful, multiyear science training programs. Our results also suggest that, while connections and enthusiasm might develop quickly, their downstream consequences might only be observed after students build stronger social relations and enthusiasm for program activities in ways that foster greater integration.