This article reports on how design-based research methodology was used to guide a line of intervention research that developed, implemented, revised, and evaluated online learning science curricula for middle school students, including general education students and English language learners (primarily of Hispanic origin). The iterative, design-based research approach was carried out in two stages with multiple steps per stage: (a) stage 1, or informed exploration, identified and described the problem under investigation; and (b) stage 2, or enactment, redesigned previously developed online science units, implemented each unit in case studies, and completed a feasibility evaluation. The present paper focuses primarily on the second stage, demonstrating the process by which online science units were repeatedly refined with input from stakeholders, and then examined for their feasibility to implement, usefulness for helping teachers engage with students, and potential to effectively deepen science knowledge. Data were drawn from multiple sources, including teacher logs, student and teacher surveys, student notebooks, content assessments, and focus groups. Results indicate that the online science units were feasible to implement, usable, and helpful. The data-driven, design-based research methodology proved to be both practical and efficacious, and underscored the critical importance of involving all stakeholders in the process of curriculum creation and refinement. This work offers a model for the development of constructivist science instructional materials for English learners using online, multimedia technology.