In this study we investigate code-crossing and multilingualism among 13–14 year olds in three schools (five classes) in the northern French city of Lille, based on data elicited during one-to-one interviews as part of a broader study of adolescent language in the city. With regard to code-crossing the study focuses on the indicative evidence of acquisition of (dialectal) Arabic by adolescents of European or Metropolitan French family background, gleaned from a series of language tests. The results suggest that for subjects of Metropolitan French background, interethnic friendships, bolstered by playful use of the language of the Other, are the single most important factor in non-institutional acquisition of Dialectal Arabic. These findings receive a degree of confirmation from the professed familiarity with a variety of Rom in one class group. Cross-ethnic language acquisition does not, however, appear to correlate in any significant way with factors that may be said to frame the socio-cultural space (such as tastes in music and style of dress) in which these teenage informants were moving at the time of the fieldwork. With regard to multilingualism subjects were found to have had exposure to a variety of European and one West-African language (Wolof). On the evidence of the language tests, the largest ethnically defined minority group, the Maghrebians manifested a range of competence in Arabic, with apparently significant differences between subjects of Algerian and Moroccan extraction.