This study introduces a new group of students to the postsecondary science agenda: latecomers to science. Latecomers, who enter postsecondary science through alternative routes because they are missing prerequisites, are less likely to graduate than traditional science students. Challenges to latecomers' persistence are explored through two questions: (1) What trends in science identity trajectories are latecomers to science able to construct during their first year in a college science program? (2) How are latecomers' identity trajectories constrained by or improvised with the cultural models and associated resources available in the figured world of a college science program? These questions are investigated through an analysis of educational activities, reflective writings, and interviews of nine latecomers. We view identification as analogous to velocity and demonstrate how recurring forces exerted by figured worlds and cultural models within them create patterns of acceleration towards or away from science, thus supporting or hindering persistence as identity trajectories gain or lose momentum. Findings show that latecomers' persistence was greatly constrained by two cultural models from the science program: good science students follow a paradigmatic sequence of courses and consistently earn good grades. Occasionally, latecomers improvised to resist these constraints. We illustrate our findings through three cases exemplifying inbound, outbound, and peripheral trends, offering a method of representing trajectories that may lead to new understandings of persistence. We also suggest implications for better supporting latecomers and connect this research to recent developments in the theoretical and methodological use of identity trajectories in understanding access to science. © 2013 Wiley Periodicals, Inc. J Res Sci Teach 50: 826–857, 2013