Through this non-experimental (Causal-Comparative) research; the written argumentative production in Spanish mother tongue (MT) of fourth semester students in university bilingual education in the undergraduate modern languages program—experimental group (EG)—was compared with that of their fourth semester monolingual university education counterparts of the Mathematics; Spanish and literature; and Biology and environmental education undergraduate programs—control group (CG)—at the University of Quindio; Colombia. It was found that the EG surpassed the GC in the written production evaluation measures of the argumentative text. Finally; it was concluded that the university bilingual education gained by the bilingual individuals during the first four semesters in the undergraduate modern languages program would explain their superiority over the monolingual individuals in the written argumentative production.