A substantial achievement gap between K-12 English learners (ELs) and non-ELs in science, technology, engineering, and mathematics (STEM) content areas exists, as indicated by national assessments of student outcomes. Considering the expected steady increase in students who are ELs in the U.S., determining methods for addressing this achievement gap is of immediate concern. Research has indicated this gap may be exacerbated by lack of adequate teacher preparation, specifically in STEM fields, to effectively teach students who are culturally and linguistically diverse (CLD). Founded in previous research about effective teacher preparation, the current case study pilots and reports on a model of early STEM preservice teacher training that integrates: knowledge of language development for ELs, early experiences with CLD learners, and professional development activities that guide the implementation of STEM pedagogical methods. Five STEM preservice teachers participated in a year-long supplemental training program focused on adapting STEM instruction for ELs. Components of the supplemental program included: (a) coursework extending teacher knowledge of EL language development, (b) fieldwork providing early exposure to research-based teaching experiences with EL students, and (c) professional development guiding the creation of hands-on STEM curriculum for diverse learners. Five secondary preservice teachers experienced increases in self-efficacy, growth in STEM instructional practices, and greater motivation for teaching in high-need schools. Results will inform educational models for improving STEM-EL teaching, thereby addressing a crucial need to serve the growing national population of underserved students.