Background: Research has shown that certification and an undergraduate major in a teaching subject are consistent positive-significant predictors of teacher effectiveness and the lack of at least a minor in an assigned subject as a consistent and negative indicator of student success. Texas secondary science teachers have either subject-specific certifications or composite science certifications. Studies revealed that a majority of prospective high school science teachers prefer to obtain certification in composite science. Certification in composite science qualifies a candidate to teach most academic science courses. However, composite teachers may be assigned to teach subjects for which they have little academic preparation. Thus, their sense of efficacy may be impacted. Several studies explored the efficacy of teachers certified in a single science subject. Minimal work examined the effectiveness of composite teachers. Studies have used scores on Personal Efficacy and Outcome Expectancy subscales in Science Teacher Efficacy Belief Instrument (STEBI) to measure teacher effectiveness. Few scales exist specifically for high school science subject teachers. Hence, STEBI was adapted for this study. Purpose: This study aims to explore the validity and reliability of a modified STEBI and to compare Personal Efficacy (PE) and Outcome Expectancy (OE) of composite-certified teachers with the subject-specific teachers. Methods: Using the adapted STEBI, PE and OE were assessed for 562 biology teachers. The validity of the instrument was examined using Principal Component Analysis (PCA) and Confirmatory Factor Analysis (CFA), and the reliability established with Cronbach's alpha. A hierarchical multiple regression analysis was used to compare PE and OE of composite teachers and subject-specific teachers controlling for undergraduate major and teaching experience. Results: The PCA supported the validity of the instrument. The subscales were reliable; alpha was 0.81 and 0.81. Certification did not predict PE or OE. The teaching experience significantly predicted personal efficacy. An undergraduate major in a teaching subject, biological science, predicted a higher level of personal efficacy and outcome expectancy than did an undergraduate major in a different field. The majority of composite teachers teach biology. Conclusion: STEBI could be adapted to study efficacy beliefs of high school subject teachers. However, subject-specific certification and composite certification did not yield significant differences. [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.]