Using evidence from a variety of sources (dialectological and sociolinguistic studies, written and oral history and works of literature), this study seeks to describe how, in a period of rapid industrial expansion to which immigrant labour was a crucial contributing factor, large numbers of Belgian migrant workers (the majority of whom were Flemish-speaking) were assimilated into the local Romance-speaking community. In an area often characterised as diglossic (French-Picard), the influx of large numbers of Flemish speakers gave rise to a three-way language-contact situation. While charting some of the most important changes in the vernaculars of Lille, the study seeks to explain why an alloctonous group of such significant proportions living so close to their homeland apparently assimilated so readily.