In the U.S., heritage Spanish speakers are those who acquire Spanish in early childhood, but then switch to English, which becomes their dominant language. Such speakers possess various degrees of bilingualism. Those of advanced proficiency have both receptive and productive oral skills in their heritage language, yet still fail to display native-like abilities in their written output. A gap in heritage language research is whether formal instruction has an impact on the linguistic competence of heritage speakers, and if so, in which areas. This case study addresses the issue by exploring the written production of advanced heritage speakers. Results after one semester of instruction revealed that a focus on explicit grammar teaching and extensive writing was beneficial. Gains after instruction were significant in orthography and general writing skills.