The authors conducted an experiment to determine if student learning in a large, upper-division developmental biology course could be enhanced by transitioning from a traditional lecture format to a more interactive classroom format. The study involved two semesters: the fall 2003 semester used the traditional lecture format, while the spring 2004 semester introduced decreased lecturing and increased student participation and cooperative problem-solving, including frequent in-class assessments. The results showed significantly higher learning gains and better conceptual understanding in the more interactive course. To assess reproducibility, the interactive course was repeated in spring 2005 with similar results. The findings suggest that incorporating interactive engagement and cooperative work into large biology courses can lead to better student learning and conceptual understanding, while retaining course content by demanding greater student responsibility for learning outside of class. The authors propose a general model for teaching large biology courses that emphasizes interactive engagement and cooperative work over traditional lecturing.The authors conducted an experiment to determine if student learning in a large, upper-division developmental biology course could be enhanced by transitioning from a traditional lecture format to a more interactive classroom format. The study involved two semesters: the fall 2003 semester used the traditional lecture format, while the spring 2004 semester introduced decreased lecturing and increased student participation and cooperative problem-solving, including frequent in-class assessments. The results showed significantly higher learning gains and better conceptual understanding in the more interactive course. To assess reproducibility, the interactive course was repeated in spring 2005 with similar results. The findings suggest that incorporating interactive engagement and cooperative work into large biology courses can lead to better student learning and conceptual understanding, while retaining course content by demanding greater student responsibility for learning outside of class. The authors propose a general model for teaching large biology courses that emphasizes interactive engagement and cooperative work over traditional lecturing.