Using Biber's (1988) multidimensional analysis, this study investigates textual variation in second language (L2) learners' writing at different proficiency levels, and attempts to identify any developmental progression. The study used a corpus of 5200 argumentative essays written by 2600 students learning English as an L2. The results indicate that advanced L2 writing is fundamentally different from less advanced L2 writing: Advanced learners' writing is closer to native speakers' written discourse, while less advanced learners' writing is closer to native speakers' spoken discourse. The patterns of development vary across different sets of textual features. "Informational" (as opposed to "involved") production and "impersonal" (as opposed to "nonimpersonal") style showed gradual development as the learners' proficiency increases. "Nonnarrative" (as opposed to "narrative") production, "elaborated" (as opposed to "situation-dependent") "reference", and "overt expression of persuasion" did not show significant differences across the proficiency levels. The article offers pedagogical implications for practices of L2 writing instruction.