Thirty-five elementary teachers participated in a yearlong professional development (PD) program whose goal was to foster science content learning while promoting language literacy for English Language Learners (ELL). The researchers utilized an explanatory design methodology to determine the degree to which science and language literacy co-developed. The research question guiding this study was: In what ways did the yearlong PD science program support teachers at 10 elementary schools to become more knowledgeable about fostering science literacy and its role in co-developing language literacy (e.g. reading, writing, listening, and speaking) for ELL? The measurable and significant gains on the quantitative state science and reading tests and the analysis of qualitative teaching episodes led to the conclusion that there is a synergy between science learning and language learning--as one increases, so does the other. (Contains 2 tables.)