Formative assessment, bilingualism, and argumentation when combined can enrich bilingual scientific literacy. However, argumentation receives little attention in the practice of bilingual science education. This article describes the effect of a formative assessment-based pedagogical strategy in promoting university students' argumentation. It examines the written and oral arguments produced by 54 undergraduates (28 females and 26 males, 16-21 years old) in Colombia during a university bilingual (Spanish-English) science course. The data used in this analysis was derived from students' written responses, and audio and video recordings. The first goal of this study was to determine how this teaching strategy could help students increase the use of English as a means of communication in argumentation in science. The second goal was to establish the potential of the strategy to engage students in argumentative classroom interactions as an essential part of formative assessment. The findings show that the strategy provided participants with opportunities to write their argumentation in Spanish, in English and in a hybrid version using code-switching. Educational implications for higher education are discussed.