This classroom-based study investigates the relative effectiveness of content and language integrated learning in Biology for students with limited knowledge of English. The sample comprised 252 German-speaking learners of English taking intermediate English courses (3rd year of English learning) at a special secondary school (German "Mittelschule), who were randomly assigned to one of three groups: lesson being conducted in English, the lesson being conducted in German, and a control group. A teaching unit about the topic "breathing" served as intervention. To determine the effects of the two instructional methods, the participants' prior knowledge about the topic and their motivation based on general interest in the subject, specific interest in the topic and the level of self-efficacy experienced was measured beforehand with pre-tests. An immediate post-test about content knowledge and motivation was taken right after the lesson, a post-test after one month should show long term consolidation or change in motivation. Even though most other studies show negative effects of bilingual teaching for students with a low level of language skills, the results here indicate that neither knowledge nor motivation in classes with relatively little knowledge of English was significantly different in both groups, students with multilingual background seemed to even benefit from the bilingual instruction provided that specially prepared lessons were used serving the learning needs of the students.