Arabic is considered a paradigmatic case of diglossia, where written language is regarded as largely the domain of fuṣḥā. Presenting the results of a large-scale survey of language attitudes and practices in Cairo, we argue that this view should be reconsidered. A representative majority of Cairo’s literate population in fact report writing predominantly in the vernacular (ʿāmmiyya), and also regard it as a legitimate written variety, contradicting common assumptions about popular language attitudes. At the same time, fuṣḥā retains its position as an idealized prestigious variety. These surprising results are explained by rising levels of literacy and the growth of computer-mediated communication. The results encourage a rethinking of the language situation in the Arab world, supporting the view that diglossia is a social and cultural resource rather than a problem. This article is in French.