"Thank you Boston. Good night and good luck," the newspaper said in a Twitter message.
Publisher Stephen M. Mindich said in a statement to staffers the "significant financial losses" at the newspaper -- which six months ago reinvented itself as a glossy magazine -- were "no longer sustainable."
"I cannot find the words to express how sad a moment this is for me," Mindich, who was with the newspaper almost since its beginning 48 years ago, said in his statement.
"We have had an extraordinary run," he said.
Sister Phoenix publications in Providence, R.I., and Portland, Maine, will stay in business, Mindich said, but WFNX.com, the Phoenix Media/Communication Corp.'s online radio station, "will not continue as it is."
He didn't say what its fate would be.
Mindich blamed the shutdown on "the economic crisis beginning in 2007 and the simultaneous radical changes in the media business, particularly as it has affected print media advertising."
About 40 employees will be let go within the week and another 10 or so will leave soon thereafter, Executive Editor Peter Kadzis told The Boston Globe.
Kadzis described the general Phoenix staffer reaction when Mindich told them the news as "shell-shocked."
Several people cried, a person at the meeting told the Globe.
The Boston Phoenix's parent company doesn't plan a formal bankruptcy filing, but hired a Boston law firm to liquidate the newspaper's assets and distribute proceeds to creditors, the Globe said.
The newspaper has $1.2 million in debts and more than $500,000 in assets, but the assets mostly consist of promised goods and services and will likely fetch significantly less than the half-million, attorney Stephen F. Gordon told the Globe.
The Phoenix was founded in 1965 by former Massachusetts Institute of Technology student newspaper editor Joe Hanlon as a four-page arts-news sheet inserted in Harvard Business School's student newspaper.
It has won numerous journalism awards, including a Pulitzer Prize in 1994 for classical music writer Lloyd Schwartz.