Justicia for Migrant Workers submitted a complaint to Office of the Independent Police Review Director on Thursday.
In October, an alleged sexual assault occurred, reportedly by a migrant worker, in Vienna, Ont., near Tillsonburg.
Police asked dozens of migrant workers for DNA samples, but not from people who matched the specific suspect description, the advocacy group claims.
The group's spokesman, Chris Ramsaroop, said the suspect was originally described as a black male in his mid to late 20s, 5-11 and muscular.
Ramsaroop said police then tested males between 21 and 61 years old, standing between five feet and 6-5 and weighing between 130 and 310 pounds.
"From talking to the workers, the only characteristic that they decided to do and engage in the volunteer — quote, unquote volunteer — sweep was around racial characteristics," Ramsaroop said.
Police stand by their investigative tactics in the case.
Last week, senior officers announced that DNA evidence had helped lead to the arrest of a migrant worker in the area.
"Criminals know no boundaries and our message today is very clear to them — neither do police," Insp. Dwight Peer said.
Sgt. Dave Rektor denied there was any racial profiling. He said investigators followed the evidence within the parameters of the law.
"We're confident in the investigative results and we stand by our investigation," Rektor said. "In this case here, it was a thorough and complete investigation and we stand behind that."
According to Ramsaroop, workers say they fear unjust prosecution in a foreign land.
Ramsaroop is also concerned about what happens to DNA samples that were not a match to the accused.
Rektor said those are destroyed by the centre of forensic science.