University bilingual science courses are on the rise because of the internationalisation of higher education. These courses are legitimate and desirable scenarios for the promotion of bilingual written scientific argumentation (BWSA) as part of bilingual scientific literacy. The problem is that very little evidence is available on how to foster university student BWSA in courses where the English (foreign language) level of the undergraduates is variable. The goal of this study was to provide evidence that collaborative learning (CL) can be used to promote BWSA in contexts where students with different levels of English proficiency study together. We examined the Spanish-English bilingual written argumentation produced by 56 undergraduates (30 females and 26 males, 16-30 years old) in Colombia during a university Spanish-English bilingual science course where the English (foreign language) level of the students was variable. The data used in this analysis was derived from students' written responses and audio recordings. The findings show that CL can contribute to BWSA as well as to specific CL skills (e.g. interdependence, individual accountability, interaction) which are important for social life. Educational implications for higher education are discussed.