Security guards told journalists from the CBC's French-language service that they had been hired around the time Wal-Mart decided to close the store in Jonquiere, citing financial reasons.
According to excerpts from a documentary to be broadcast Friday on the programme Zone Libre, union leaders and workers sympathetic to the drive were targeted by undercover security guards.
One former guard said he patrolled the store in civilian clothes, watching employees.
Another agent said the store's surveillance cameras were used to follow certain employees.
The guards said they're tasks didn't correspond to their "normal'' duties.
Wal-Mart Canada president and CEO Mario Pilozzi denied the allegations.
"No, we wouldn't tolerate the situation you mentioned,'' Pilozzi told Radio-Canada. "No idea about what you're talking about.''
Spying on union leaders or sympathizers is illegal under the Quebec Labour Code.