Social scientists have long recognized field of study as an important mechanism of gender differentiation and stratification in U.S. higher education, but they have rarely attended to how elective curriculums mediate gender differentiation in major selection. Under elective curriculums, major selection is an iterative process, in which students select courses in stepwise fashion at the beginning of each academic term, and are able to change majors early in their undergraduate careers. We observe how an elective curriculum mediates gendered patterns of major selection, using a novel data set describing 11,730 students at a large public research university. We find (a) gender and intended major are strongly correlated upon college entry; (b) large proportions of students change majors between entry and declaration; (c) because most changes are to academically adjacent fields, gendered patterns in field of study persist through the undergraduate career. Findings suggest the value of an iterative conception of major selection and offer tractable means for intervening in the process through which students select majors.