The BLS survey showed 16.28 million wage and salary workers were union members last year, virtually flat from the 16.26 million figure the year before.
Of the worker segments, about 10% of private-sector workers held union cards while those working for a government agency were far more likely to belong, the BLS said.
In particular, police officers, firefighters and other protective-service workers had the highest unionization rate, about 38%.
The union membership rate has fallen from a high of 20.1% in 1983, the first year for which comparable data are available, the bureau said.
Among demographic groups, the BLS noted that blacks were more likely to be union members, at 17% than whites (13.1%) or Hispanics (11.3%).