The current short-term longitudinal study investigated the role of college students' identity development and motivational beliefs in predicting their chemistry achievement and intentions to leave science, technology, engineering, and math (STEM) majors. We collected 4 waves of data over 1 semester from 363 diverse undergraduate STEM students enrolled in a chemistry lab course. The results of analyses that examined the reciprocal relations among the variables over time (i.e., cross-lagged path analysis) suggested that identity development that involved exploration (e.g., information-seeking, reflection) was positively related to students' beliefs about their competence and value for the STEM major and negatively related to perceptions of effort cost (drawbacks associated with time and effort) for the major. Identity development that did not involve exploration was related to low competence beliefs and high perceptions of costs for the STEM major. Competence beliefs, values, and perceptions of cost for the major were dynamically related to chemistry achievement and to students' intentions to leave the STEM major over the semester, with different kinds of cost perceptions (drawbacks associated with effort, lost opportunities, and stress and anxiety) relating differentially to students' intentions. The results support the role of identity development in students' motivation for a STEM major and address a gap in the literature regarding the role of perceived cost in students' academic choices.