<![CDATA[Effective biology learning requires a teaching style that is able to actively build conceptual understanding. However, the reality in many Indonesian schools, including private madrasah schools with limited facilities, is still dominated by a teacher-centered approach that positions students as passive recipients of information, while in-depth qualitative studies on constructivist-based biology teachers' teaching styles in such contexts remain very limited. This study aims to describe the biology teacher's teaching style, identify the learning theories applied, determine the most dominant theory, and analyze its relationship with students' activeness and conceptual understanding at MAS Ummi Kalsum, Gunungsitoli City. The study employed a descriptive qualitative approach of the field research type with one biology teacher as the sole subject selected purposively; data were collected through non-participant observation, semi-structured interviews, a twenty-item Likert-scale questionnaire, and documentation, then analyzed using the Miles and Huberman model and validated through source and method triangulation. The results showed that the teacher applied a student-centered teaching style with the dominance of the Problem-Based Learning method, obtaining an average questionnaire score of 4.70 on a 5-point scale (15 out of 20 indicators were rated strongly agree). Constructivism was proven to be the most dominant foundation, complementarily supported by cognitive, behaviorist, and humanistic theories that were organically integrated and contributed significantly to the high level of student activeness and conceptual understanding.]]>
