In this paper we analyze the impact of an increase in the local supply of immigrants on firms’ outcomes, allowing for heterogeneous effects across firms. Using micro-level data on French manufacturing firms spanning the period 1995–2005, we show that a supply-driven increase in the share of foreign-born workers in a French département (a small geographic area) increased the total factor productivity of firms in that département. We also find this effect to be significantly stronger for firms with low initial productivity and small size. The positive productivity effect of immigrants was also associated with faster growth of capital, larger exports, and higher wages for natives. Highly-skilled natives, in competition with immigrants, moved towards firms hiring fewer immigrants spreading positive productivity effects to those firms too.