This study examines the within-field persistence rates of physical science/engineering (PS/E) students compared with those of students in life science, business, social science, education, and humanities. The data are drawn from a nationally representative sample of college matriculates and include students' high school and college transcript information. PS/E students are the most academically prepared for college, as measured by high school mathematics and English performance, yet they earn lower within-field college grades than students earn in other fields. The results of logistic regression analyses show that the gap in college grades, net of background and preparation, partly explains why students in business, education, and humanities have higher persistence rates than students in PS/E. Moreover, the association between within-field college grades and persistence is strongest for PS/E students. Taken together, these results provide evidence that differences in grading across fields contribute to the relatively low persistence rates found among PS/E students.