Teacher content knowledge has been identified as a key prerequisite to effective instruction, and current educational policies require measurement of teacher content knowledge to assess candidacy for licensure. The primary instruments used in the United States are the Praxis Subject Assessment exams, which are designed to measure the subject-specific content knowledge needed to be a teacher. The Praxis Biology Subject Assessment exam, used by 42 U.S. states in the past decade, is the most common national measure used to determine biology content knowledge for teacher certification. Demographic and performance data from examinees (N = 43,798) who took the Praxis Biology Subject Assessment from 2006 to 2015 were compared to present a much-needed picture of who is seeking certification to teach biology, how different groups of aspiring biology teachers have performed, and how demographic makeup of prospective biology teachers compares with reports in previous studies describing the composition of the biology teacher workforce. Results indicate the majority of students self-reported as White (76%), female (66%), having undergraduate grade point averages (GPAs) at or above a 3.0 (76%) and majoring in biology (45%). Additionally, the demographic data were included in a linear regression model to determine the factors that explained the most variance in performance of the examinee. The model revealed substantial differences in average performance and pass rates between examinees of different genders, races, undergraduate majors, undergraduate GPAs, and census regions. This suggests that if the examinee is a White science, technology, engineering, and mathematics major, man with a 3.5 or higher undergraduate GPA, resides in the western United States, or plans to teach in a suburban school, the examinee will on average outperform their counterparts on the exam. From our analyses, we suggest several measures for the improvement of the biology teaching workforce and establish potential issues in the teacher pipeline that may impact the quality and diversity of U.S. biology teachers.