The "Filière Bilingue" (FiBi) is a Swiss public two-way immersion program integrating French-speaking, German-speaking and allophone learners (having neither French nor German as an L1) in each class. Located in the bilingual city of Biel/ Bienne, the "diglossic" situation represented by the use of Swiss German and standard German results in francophone students learning both the dialect and the standard language up to a certain degree. In school, standard German is gradually introduced and exclusively used to teaching from Grade 1 on. Our longitudinal study reports findings from statistical analyses of FiBi learners' oral proficiency development in French and German (n = 138) and their monolingually taught peers (n = 94) by means of 1173 interviews during their first four years (K-4). Findings show that FiBi learners' performance in their registration language is significantly higher than in control classes during Kindergarten. Allophone learners' performance of both groups impressively increases but FiBi allophone learners' performance appears to be significantly higher compared to their monolingually taught peers. Regarding the partner language, performance in French of K-4 German-speaking FiBi learners impressively improves. Probably due to the diglossia, performance in German of French-speaking FiBi learners also significantly increases although less impressively. To conclude specific pedagogical implications are discussed.