The role of English as a first language in teaching-learning process of Portuguese as a second language for foreigners investigates, as the own title indicates, the relation between first and second languages - L1 and L2 - in the process of L2 development by adult speakers. The main objective of the dissertation was to confront teaching theories and methods that defend the monolinguism in L2 classroom, and seek to distance L2 from L1, that is perceived as interference in the learning process. Questioning this idea, the research analyzed a period of ten hours of Portuguese as L2 classes, taught for native English speakers, in an immersion context in Brazil, specifically in PUC-Rio. To develop this analysis, basics concepts of psychoanalysis, interculturalism and social construction of knowledge were used as theoretical foundation, that shed light on L1-L2 interface, involving all of its dimensions, especially the subjective, the cultural and the social ones, respectively. After the classes transcriptions, the research identified the moments in which the students switched from Portuguese to English, observing the reasons and consequences of this alternation from L2 to L1. As from this, we concluded that students used English, their mother language, with four different functions: expressing feelings, evaluating, understanding structure and expanding vocabulary. Excepting for the first one, the three others resulted in knowledge construction and, thus, it manifested itself as a pedagogical resource. In its turn, the L1 use for expressing feelings in L2 classroom was noted in a different way, because it was not effectively positive to the learning process. Therefore, the information provided by the analysis of data indicated a positive tendency in the L1 use, corroborating the thesis that L1 can be an auxiliary resource for new language learning.