“I’ve seen him work 12 hours without a break,” said Ahmad Karimi-Hakkak, director of the Center for Persian Studies at the University of Maryland, who has known Mr. Yarshater for more than 40 years. He remembers a visit when Mr. Yarshater stayed up until 3 a.m. editing. Three hours later, he was in the shower, getting ready to return to work.

Mr. Yarshater expects others to have equal enthusiasm for the task. It took him 17 years to choose his replacements, rejecting one potential successor when he concluded that the man was “too concerned about the number of holidays he could take and the number of hours he would work.”

Now Mr. Yarshater works only until 9 p.m., staying long after his colleagues have turned off their lights. When he returns home, he indulges in his latest hobby: learning Russian.

The 1,480 contributors from around the world who, so far, have composed 6,500 entries are familiar with Mr. Yarshater’s relentlessness. “By hook or by crook, he gets you to do what he wants you to do,” Mr. Karimi-Hakkak said. (Eight hundred entries out of alphabetical order are posted in an online version.)

The managing editor, Ahmad Ashraf, said he spent a year working on an entry on social class. He received a $1,000 honorarium for his effort. “We are working here on half salary,” he said. “This is just a love of the work.”

Editing can be brutal. Until recently Mr. Yarshater meticulously checked and revised every entry. “Maybe I’m a faultfinder,” he conceded. He tries to praise colleagues and assistants, but said “it is not in my nature.”