The vote came two weeks after four union leaders were fired - they say because of their union activism.
"We're supposed to be security officers, yet we are afraid," said Juanita Love, who was a museum guard for five years. She said Roman fired her a few days after she spoke at a union rally Oct. 8.
Roman Sentry, of Philadelphia, would not comment on Love or the election.
AlliedBarton spokesman Larry Rubin said he couldn't comment on personnel matters. Some of the four who were fired were AlliedBarton employees.
Union advocates say it is typical - and the National Labor Relations Act says it is illegal - for companies to try to erode support for a union by firing activists and leaders. Companies usually say they have a legitimate reason, such as tardiness.
The Philadelphia Museum of Art, a city-related institution, has contracted out its security work since 1992. Formerly city employees who earned about $18 an hour, the guards now bring home $10.03 an hour.
Most of the guards, about 130, according to the union, work for AlliedBarton.
Local financier Ronald O. Perelman serves on the AlliedBarton board. The museum's new annex, the Ruth and Raymond G. Perelman building, is named after his parents.
Love said she had been dismissed for abandoning her post on the day of the rally. She said that a supervisor had given permission for her to leave because the museum was overstaffed.
Contact staff writer Jane M. Von Bergen at 215-854-2769 or firstname.lastname@example.org.