This article reports two intervention studies testing the effects of plurilingual teaching on grammatical development among primary-school students learning English as a foreign language (FL). In a pre-posttest control-group design, more than 200 9-10-year old majority language German and minority language students received plurilingual FL teaching (intervention group) or regular FL-only teaching (control group). Study 1 on the acquisition of "wh"-questions showed that systematic cross-linguistic comparisons of the FL with the majority language and minority languages facilitate acquisition of object questions. In Study 2 on passives, the intervention and the control groups both demonstrated comparable gains. We suggest that plurilingual teaching has advantages when the majority language differs from the target language (Study 1) yet not when a phenomenon is comparable across languages (Study 2). In neither study did learners show generalization to related grammatical phenomena. Finally, majority language and minority language students did not perform differently, which suggests that plurilingual FL teaching is suitable for all FL learners. These findings demonstrate that plurilingual FL teaching facilitates grammatical development by increasing learners' awareness of cross-linguistic similarities and differences.