In 2003, the French government decided to reform the system managing the different minimum wage regulations and the targeted reductions to employers' social security contributions. The main objective of the reform was to simplify the complex regulations that were created by the progressive introduction of the 35-hour week. The reform incidentally created large variations in labour costs, depending on the type of firm and the wage level within the firm. This paper presents an evaluation of the impact of this reform on employment using a balanced panel of firms with more than five employees, drawn from a matching between several administrative data sources from 2000 to 2005. In both types of firm, significant employment elasticities can be found with respect to labour costs that have the expected signs: a rise of 1% in average labour costs reduces employment by 0.4%. As the majority of firms that remained on the 39-hour week received greater reductions, the Fillon reform allowed them to raise their level of employment. Firms that adopted the 35-hour week experienced the opposite: the reform led to a fall in employment. Ultimately, the Fillon reform has had no clear effect on aggregate employment, measured either in job numbers or full-time equivalents (FTE).