Thirty-five elementary teachers participated in a yearlong professional development (PD) program that was designed to foster a culture of on-going teacher learning to promote the co-development of science and language literacy for English language learners (ELL). An explanatory design methodology was used 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 mandated state science and reading tests and the analysis of qualitative teaching episodes led to the conclusion that demonstrated the synergy between science learning and language learning--as one increased, so did the other.