This paper addresses two related issues: the effect of the ‘regulatory shock’ of the National Minimum Wage on small firms and the consequent effects on the commonly observed practice of ‘informality’. It draws on a survey of such firms but primarily uses case study evidence from five firms to examine the processes at work. Detailed case studies of such firms remain rare, and they tend to analyse a largely unchanging world. We show how patterns of informality change over time. The regulatory shock had varying effects, as some firms moved up-market and others were pressed to the edges of the legitimate market and in some cases out of business altogether. In the firms moving up-market, informality was redefined into a more disciplined and formalised approach, while those going down-market relied even more heavily on family and other ties to survive. The NMW led to some improved wages, though there was also evasion, and it has sharpened the divide between the legitimate and illicit areas of business.