Despite all the national efforts to increase STEM enrollment in the United States, the gap between the U.S. and other developed countries in terms of STEM graduates has widened over the last 20 years. Researchers have studied factors such as gender, race, high school GPA, and the student's socioeconomic status for their impact on STEM enrollment. This study offers another possible explanation of why students might choose, or not choose, to enroll in STEM majors by examining the relationship between personality and STEM enrollment. The sample included 2,745 respondents to the 2008 Cooperative Institutional Research Program freshman survey at a large research university in the southwestern United States. Factor analysis was used to create four personality scales, based on John Holland's theory of personality types, with items selected from the survey. Logistic regression was utilized to answer three research questions: Are students classified as a strong investigative personality type more likely to enroll in STEM majors than students classified as a weak investigative personality type? Are there differences in their likelihood to enroll in STEM majors among students of investigative-social, investigative-artistic, and investigative-enterprising personality types? What effect does personality have on students' self-selection into a biological versus a physical STEM major? Results suggested that students with a combined investigative and social personality were more likely to enroll in STEM majors whereas students with a combined investigative and artistic personality were less likely to do so. Additionally, STEM students with an enterprising personality were more likely to choose a biological STEM major than a physical STEM major. These results should benefit educators and policy makers who seek to strengthen the pipeline into STEM fields. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]