Enabling learners to successfully use their second language (L2) in meaningful ways is a critical goal of instruction. Ultimately, most learners want to meet the L2 demands of the contexts in which they will use the language. To accomplish this, learners must develop linguistic knowledge and apply it in a manner that is contextually appropriate considering the requirements of the task at hand. In other words, learners must develop their L2 across both linguistic and communicative aspects to use it successfully. However, there is a paucity of L2 research in which linguistic and communicative performance have been simultaneously investigated.In this study, I investigated the development of English as a Foreign Language (EFL) learners’ L2 written production across the lexical and communicative dimensions of performance. The study involved 290 Japanese participants recruited from 20 intact EFL classes at four tertiary educational institutions in Japan, representing a wide range of L2 English proficiency levels and instructional contexts. The study used a non-intervention, repeated-measures design, allowing for the general development of participants’ L2 English writing to be examined. Participants’ L2 English written responses were collected using four argumentative writing tasks, which were administered at the beginning and end of the first and second semesters in a counterbalanced manner. Although a total of 952 responses were collected, responses shorter than 50 words were removed, leaving a total of 775 responses written by 250 participants. The 775 responses included in the primary analyses constituted a corpus of 89,122 words. Twenty-four single-word, multi-word, and lexical variation measures were calculated for the responses and subjected to an exploratory factor analysis. Seven latent lexical factors were identified in the data: High-Frequency Trigrams, Lexical Clarity, High-Frequency Bigrams, Lexical Variation, Lexical Breadth, Low-Frequency N-Grams, and Directional Association Strength of N-Grams. In addition, raters scored the responses for functional adequacy (i.e., Content, Comprehensibility, Organization, and Task Completion) and Lexical Appropriateness. The scores were analyzed using many-facet Rasch measurement, which converted the ordinal scores into equal-interval measures that had been adjusted for the influences of task and rater severity. The lexical factor scores and communicative Rasch measures were examined using linear mixed modeling, dominance analysis, and latent growth modeling to investigate (a) if and how lexical development had occurred, (b) if and how communicative development had occurred, (c) the relationships between the lexical and communicative components, and (d) the relationship between lexical and communicative growth. For the lexical factors, the results indicated that Directional Association Strength of N-Grams scores increased in a linear manner. Directional Association Strength of N-Grams comprised ΔP scores, which indicate the degree to which the first word(s) are predictive of the following word(s) in two- and three-word combinations. Thus, the results indicated that participants’ use of multi-word expressions improved. On the other hand, Lexical Clarity, which comprised imageability, concreteness, meaningfulness, and hypernymy scores, showed quadratic change, with scores improving and then regressing. Thus, the findings provide evidence of differing developmental trends for lexical aspects of L2 writing. For the communicative measures, the results indicated that Comprehensibility, Organization, and Lexical Appropriateness changed substantially over time. Improvement of Task Completion was dependent on the university context, and little change was observed for Content. Lexical Appropriateness showed the most improvement, with evidence of both linear and quadratic change. Bias interaction analyses also confirmed the presence of linear and quadratic trends for the communicative measures. Thus, the findings provide evidence of differing developmental trends for communicative aspects of L2 writing. For the relationships between the lexical and communicative components, the results indicated that two lexical factors were of key importance: Lexical Variation and Directional Association Strength of N-Grams. Lexical Variation was found to predict Content, Organization, and Task Completion; and Directional Association Strength of N-Grams was found to predict Comprehensibility and Lexical Appropriateness. The findings suggest that L2 performance assessments should not conflate measurement of lexical variation and use of multi-word expressions because they diverge in terms of the communicative outcomes they predict. The results also indicated a positive relationship between lexical and communicative development. A parallel process latent growth model was constructed that related lexical and communicative growth. The paths tested in the model suggest participants who had lower initial communicative scores were able to increase their lexical scores at faster rates, which in turn leveraged communicative growth. The findings highlight the potential for learners to improve their communicative ability through a targeted focus on multi-word expressions. / Applied Linguistics