Using a sequential, explanatory mixed methods design, this dissertation study compared students who persist in the biology major (persisters) with students who leave the biology major (switchers) in terms of how their pre-college experiences, college biology experiences, and biology performance figured into their choice of biology and their persistence in or departure from the biology major. This study combined 1) quantitative comparisons of biology persisters and switchers via a questionnaire developed for the study and survival analysis of a larger population of biology freshmen with 2) qualitative comparison of biology switchers and persisters via semi-structured life story interviews and homogenous focus groups. 319 students (207 persisters and 112 switchers) participated in the questionnaire and 36 students (20 persisters and 16 switchers) participated in life story and focus group interviews. All participants were undergraduates who entered The University of Texas at Austin as biology freshmen in the fall semesters of 2000 through 2004. Findings of this study suggest: 1) Regardless of eventual major, biology students enter college with generally the same suite of experiences, sources of personal encouragement, and reasons for choosing the biology major; 2) Despite the fact that they have also had poor experiences in the major, biology persisters do not actively decide to stay in the biology major; they simply do not leave; 3) Based upon survival analysis, biology students are most at-risk of leaving the biology major during the first two years of college and if they are African-American or Latino, women, or seeking a Bachelor of Arts degree (rather than a Bachelor of Science); 4) Biology switchers do not leave biology due to preference for other disciplines; they leave due to difficulties or dissatisfaction with aspects of the biology major, including their courses, faculty, and peers; 5) Biology performance has a differential effect on persistence in the biology major, depending on how well students perform in comparison to other courses or other students. / text