Until 1792, marine insurance in America was entirely in the hands of private underwriters (although American merchants also obtained some insurance in London). By 1810, the business had shifted into the hands of joint-stock corporations (and much less was being done in London). This paper investigates this rapid transition between institutional forms, focusing on a pivotal period when newly formed corporations existed side by side with private underwriters, and when the risks to American shipping increased substantially as a result of French privateering during the Quasi-War between the United States and France. I use data from a private underwriter and a marine insurance corporation to examine the Quasi-War's effect on insurance rates, the volume of business, and the institutional structure of the marine insurance industry. I also contrast the American experience with that of Britain, where a similar transition to corporate underwriting did not occur at this time despite the stimulus of the Napoleonic wars, and with France and Holland, where a more gradual transition occurred.