From the perspective of social cognitive theory, the motivation of students to learn science in college courses was examined. The students--367 science majors and 313 nonscience majors--responded to the Science Motivation Questionnaire II, which assessed five motivation components: intrinsic motivation, self-determination, self-efficacy, career motivation, and grade motivation. Exploratory and confirmatory factor analyses provided evidence of questionnaire construct validity. The motivation components, especially self-efficacy, were related to the students' college science grade point averages. The science majors scored higher than the nonscience majors on all of the motivation components. Among both science majors and nonscience majors, men had higher self-efficacy than women, and women had higher self-determination than men. The findings suggest that the questionnaire is a valid and efficient tool for assessing components of students' motivation to learn science in college courses, and that the components play a role in students' science achievement. (Contains 3 tables and 1 figure.)