In recent years, the issue of using bilingual education has come to the public's attention. A widespread belief is that students in bilingual classes achieve a higher competence in the language, but that they lag behind in subject-specific knowledge when compared to fellow students in traditional (i.e. monolingual) classroom settings. Nevertheless, by evaluating knowledge gain in a short teaching unit, previous studies have shown that bilingual students rarely experience drawbacks. Although there are a variety of different opinions, the present study aims at detecting how subject-specific knowledge is influenced by bilingual biology lessons. The study was conducted at a grammar school in North Rhine-Westphalia, Germany, with 13- and 14-year-old students. To assess differences in students' knowledge gain, test and control groups were used, in which students were taught 12 identical biology lessons in a bilingual and a traditional classroom. The unit consisted of basic characteristics seen in (social) insects. Both classes took a knowledge test before and after the unit. We hypothesised that there would be a higher increase in knowledge for the bilingual class compared to the monolingual class. Results suggested a slightly higher knowledge gain in bilingual students, but no significant differences were revealed.