English dominates as an international language for science and research. Consequently, this is the most widely used language in university bilingual (e.g. Spanish–English, Chinese–English, Swedish–English) science courses around the world. However, very little is known about how to foster bilingual scientific writing in these types of courses. This article describes the effect of a systematic and purposeful code-switching pedagogical strategy in fostering undergraduates' bilingual scientific writing. Participants were 76 students (41 female and 35 male, 17–23 years old) enrolled in a university bilingual (Spanish–English) Biology course in Colombia. The data used in this analysis were derived from students' argumentative written productions, and audio and video recordings. The findings show that this systematic and purposeful code-switching pedagogical strategy was not only a useful basis for promoting students' bilingual scientific writing but also a valuable way for students to gain confidence in their foreign language written skills.