The development of lexical complexity in second language acquisition has received a considerable amount of attention in applied linguistics research. Many studies have examined the role of lexical diversity, sophistication and density as indicators of L2 proficiency. Few studies, though, have considered the development of lexical complexity from an explicitly cross-linguistic perspective. This article reports on an explorative, cross-linguistic study on the development of lexical diversity, sophistication and density in L2 French and English at four levels of linguistic proficiency. Additionally, the study proposes a number of alternative measures tapping into collocational knowledge and lexical sophistication. The analyses were carried out on a cross-sectional, multilingual corpus of L2 French and English consisting of oral narrative data. The results show a similar development of lexical diversity in L2 French and English, but considerably different developmental tendencies in terms of sophistication and density. The concluding sections discuss possible explanations for these differences and consequences for the measurement of linguistic proficiency.