This study explored the relationships between English language proficiency and performance in online, asynchronous science courses. Each participant completed a Language Background Questionnaire (LBQ) to indicate if English was not their native language, thus categorizing them as an English Language Learner (ELL). The Combined English Language Skills Assessment (CELSA) measured English language proficiency. Of 25 ELLs 13 were interviewed to elicit their perspectives of learning strategies used in an online environment. Quantitative analyses involved t-tests and Pearson's correlation among 91 participants' CELSA scores and final exam results. For non-ELL students, there were weak to moderate, positive correlations for Bacteriology 2: r(57)=0.47, p<0.00, Clinical Chemistry 1: r(33)=0.36, p=0.04, and Clinical Chemistry 2: r(33)=0.44, p=0.01; no other significant correlations were found. This study may inform interventions and strategies that enhance learning and performance in science-based coursework in courses taught in asynchronous, online programs for all students, particularly ELLs.