The current study compared the effects of two second language (L2) instruction types--processing instruction (VanPatten, 2004) and traditional output-based instruction--on the development of the Spanish past subjunctive among U.S. Spanish heritage language speakers and traditional L2 learners. After exposure to instruction, both the heritage learners and the L2 learners showed significant improvement on interpretation and production tasks. Only the L2 learners showed significant improvement for grammaticality judgments. Overall, the L2 learners outperformed the heritage learners. The results suggest that heritage speakers' language development may differ from that of L2 learners, although they also suggest that heritage speakers can benefit from focused grammar instruction.