Recent multilingual perspectives view second language writing as part of a language user's dynamic multilingual repertoire, always occurring in the (cognitive and social) context of some first language(s), and often of first language literacy and/or use of other languages. This paper focuses on one element of written language (lexical sophistication), and examines preliminary findings about this variable across languages in a sample of undergraduate student writers in two quite divergent languages: Arabic (L1) and English (L2). Significant differences were found in students' use of vocabulary from different frequency bands of vocabulary in their L1 writing, their writing in L2, and a sample of expert native speaker writing in each language. There was, however, little evidence for development in lexical sophistication in either language over sixteen weeks of a semester. Overall, lexical sophistication did not seem to bear a clear relation to academic achievement in either writing course, nor to previous language learning experience, except in one measure of English vocabulary. There is the possibility of a positive correlation between lexical sophistication in the two languages, although this was not significant in this study. Implications for further research are discussed.