The purpose of the study is to understand secondary science teachers' self-efficacy beliefs and implementation of inquiry to provide information for the improvement of science education in a Midwestern U.S. state that has not yet adopted NGSS. An explanatory sequential mixed methods design was selected to capture both a broad perspective of science teachers' self-efficacy and inquiry implementation. Thirty-nine secondary science teachers participated in this study. The Teaching Science as Inquiry questionnaire (TSI) was employed, and the online survey software Qualtrics was used to collect quantitative data. For the qualitative phase, data collection consisted of 13 face-to-face, individual, audio-taped interviews. Findings demonstrate that there is a connection between the teachers' self-efficacy beliefs and implementation of all five essential features of inquiry. The teachers are also more confident in teaching science through teacher-centered inquiry than student-centered inquiry. Additionally, teachers' background in terms of content knowledge, pedagogical knowledge, and experience is a variable that affects teachers' self-efficacy and implementation of teaching science as inquiry.