This study has two research purposes related to L2 writing development. The first aim of this study is to explore how L2 learners receiving intensive ESL instruction develop their writing proficiency (writing quality from an analytic rubric) across two writing tasks (narrative and argumentative writing) over one semester. Limiting its focus to written lexical proficiency, the study also examines how lexical sophistication measures contribute to expert ratings of vocabulary scores in each task type. For this study I examined a total of 306 independent task essays. The results indicated that the traits of content, organization, and vocabulary improved significantly over time, whereas those of language use and mechanics did not. The learners' narrative essays received higher scores than their argumentative essays, but the patterns of writing development over one semester did not differ across the two tasks. For lexical proficiency, this study showed that vocabulary scores of narrative and argumentative writing were best predicted by different combinations of lexical sophistication measures; the regression models of the two writing tasks indicated some common lexical features and task-specific ones. The findings suggest implications for writing assessment and pedagogy.