This article examines the acquisition of German as the weaker language in the cases of German-Polish bilingual children. Focusing on negation and verb position, phenomena that have frequently been taken as diagnostic when distinguishing between the course of language development characteristic for first (L1) and second language acquisition (L2), we analyse experimental and productive data from six simultaneously bilingual children. Due to the constrained input, German is their weaker language. The results in Forced Choice and Grammaticality Judgements tasks are compared with the results of monolingual children. We show that in the area of negation the acquisition of German as the weaker language resembles L1, and in the area of inversion and verb final position the development of the weaker language is delayed. The striking difference between bilinguals’ results in the experimental vs. productive tasks points to specific processing mechanisms in bilingual language use. In narrative contexts of the production tasks the language of the performance is activated, while the other is inhibited, which leads to a target-like performance. Structural properties of the stronger language tend to be activated, however, in the experimental tasks involving the weaker language, resulting in non-target-like responses.