This study investigates trajectories of L2 development in writing in two genres over multiple time points in a semester-long ESL writing program. Conceptually replicating previous studies on genre and development, we analyzed 270 argumentative and narrative essays written by 45 ESL students to find developmental and genre differences, using fine-grained indices that tap into multiple dimensions of complexity. We found a genre effect on most complexity features, with significant differences between the genres. We also present evidence of growth in linguistic complexity, reporting linear developmental trajectories for lexical features but not for clausal or phrasal features. In addition, we found evidence of interaction effects between genre and time for some indices that are sensitive to unique combinations of genre and development. We conclude that development in L2 writing involves the nonlinear emergence of linguistic usages over time and that the developmental pattern can be distinct for different genres.