Janesville police said they have arrested two people suspected of taking kitchen grease from trash bins outside restaurants.
A DeForest company that pays restaurants for their grease said the thieves are stealing from businesses.
Police said that once thieves get their hands on kitchen grease, the goal is to make deals with people hoping to produce biofuels or refine the oil illegally.
At The Ground Round in Janesville, the demand for new kitchen grease prompts a regular task.
"And we'll dump a fryer on Thursday and a fryer on Sunday," said Ground Round manager Tom Widner.
But what the kitchen may see as trash, more thieves view as an opportunity.
"So, right now, they're stealing from the restaurant, and they're also stealing from us," said Brian Lodding, transportation manager with Sanimax, who says the grease belongs to his company.
The business pays restaurants for their kitchen grease, all to recycle and develop into biofuels.
"I've been in the business 33 years. This is the first time we've seen theft at this magnitude," Lodding said.
Sanimax says recent thefts have cut into nearly a quarter of its annual grease collection and that tracking thieves has been a challenge.
The company said there was a break in the case in the overnight hours of Wednesday morning in Janesville.
"One of our drivers pulled into the Wendy's on Milton Avenue, and they saw what we believed was a truck that's been the area stealing grease," Lodding said.
"We may drive by restaurants, we know they come and get that in the middle of the night," said Janesville police Officer Chad Sullivan. "It wouldn't be suspicious to us. So that person catching them in the act, calling dispatch, got us in the area quickly."
Police said David Clark, 21, and Enrique Jaime, 28, were arrested on theft charges. Police said Clark and Jaime are suspected of stealing $370 worth of restaurant grease.
Sanimax said he hopes the arrests prompt efforts to find the root of problem.
"So you almost want to go after the bigger guy rather than the guy that's actually picking it up," Lodding said.
Sanimax said the grease is worth about 40 cents a pound and most restaurants receive about $100 to $200 with every collection, usually every other month.
Janesville police said they reported the recent kitchen grease arrests to the Wisconsin Crime Alert Network to help notify other law enforcement agencies of these thefts.