About 56.5 million people watched Barack Obama's surprise TV appearance Sunday night announcing the killing of Osama bin Laden. It was the president's biggest TV audience since he was elected, according to Nielsen Co.

Mr. Obama's speech was carried on nine TV networks from 11:35 p.m. to 11:44 p.m. Eastern time and watched by 57% of U.S. households with their TVs on in those nine minutes, Nielsen said. By the time Mr. Obama took the air, reports had been leaking out for more than an hour that bin Laden had been killed by U.S....