Using longitudinal administrative data from a large elite research university, this paper separately analyzes the determinants of persistence for life and physical science majors. My results confirm much of the previous research on major persistence in the sciences, but I document that many findings are solely driven by persistence patterns in the physical sciences. For example, I show that the previously documented gender gap in science major persistence is due entirely to a large gap in the physical sciences. Despite large differences in persistence patterns between physical and life science, persistence in both fields is similarly influenced by grades. I provide suggestive evidence that students in both fields are "pulled away" by their high grades in non-science courses and "pushed out" by their low grades in their major field. In the physical sciences, analyses using within course and cohort variation show that peer quality in a student's introductory courses has a lasting impact on the probability of persisting. (Contains 7 tables, 10 figures and 12 footnotes.)