Pre-print, Working paper: This paper assesses the causal effect of partial employment protection on workers' subjective job security via the perceived probability of layoff. We consider the rise in the French Delalande tax, which is paid by private firms if they lay off older workers. This reform was restricted to large firms and therefore allows us to use a difference-indifference strategy. In ECHP data, we find that the change in the perceived probability of layoffs induced by the higher Delalande tax improved the subjective job security of older (protected) workers, but at the cost of a negative externality on younger (unprotected) workers. While the fall in perceived job security of younger workers is mirrored by actual increase in their layoff rate, the rise in perceived job security of older workers is more puzzling because we do not observe a reduction in their layoff rate.