We empirically document a strategy through which corporations avoid state income taxes. Examining aggregated American state and Canadian provincial data from 1983–1991, we find corporate income tax revenues are concave in corporate tax rates, consistent with firms shifting their tax bases to more favourably taxed jurisdictions. Additional tests exploit unique features of state formula apportionment systems and find manufacturing shipments from states that tax outside their borders (throwback states) are decreasing in corporate income tax rates on sales.