Restaurant-goers ordering fish at Valley eateries should proceed with caution, as they may be paying for one food but eating something entirely different.

The 5 Investigates Team went undercover and found several restaurants breaking the law.

Chef Cullen Campbell has worked in fine kitchens for 11 years and said he's familiar with restaurants switching out fish -- listing one type on the menu while serving another.

"When I've seen it done, nobody has even mentioned that, you know, maybe it's a different fish," Campbell said.

That sort of swapping is illegal, and most customers will never know when it happens to them.

The National Seafood Inspection Laboratory found that 37 percent of fish sold is mislabeled, meaning one species is substituted for another.

With red snapper, the mislabeling happens 80 percent of the time.

The 5 Investigates went undercover at five Valley restaurants, some upscale and some "down home." At each restaurant, the 5 Investigates producer ordered red snapper and had it boxed up to go. The 5 Investigates Team then froze, wrapped and labeled the fish, and sent it to a lab in New York for DNA testing.

The Fish Market in Phoenix boasts the freshest, highest-quality seafood, but DNA results showed the red snapper they sold was actually grouper, a much cheaper fish.

Aiden Coburn, quality control director for the Fish Market, sent a written statement to the 5 Investigates saying, "I see it as an honest mistake... Whoever was behind the grill that night picked up the wrong portion, cooked it, and sent it out there. Thank you for exposing it."

The 5 Investigates, however, found that grouper is not even listed on the restaurant's menu.

The red snapper ordered at McGrath's Fish House in Mesa also came back as grouper.

When the 5 Investigates asked managers about the results, they admitted that they knew it wasn't red snapper. They insisted that it was rockfish, a less expensive look-alike.

The vice president of McGrath's told the 5 Investigates over the phone that the restaurant used to list rockfish on the menu, but when sales dropped, the dish's name was changed to a more popular one -- red snapper.

"It's a whole recognition thing," Campbell said. "That's really the main reason, is just to help it sell."

The 5 Investigates also visited Lo Lo's Chicken and Waffles, a popular mom and pop shop in central Phoenix.

On most days, customers have to fight for a table.

What they don't know, though, is that the red snapper they served could not be identified by the DNA lab in New York.

The lab's official finding was that it was an unknown species.

The owner admitted to the 5 Investigates that he knew the restaurant wasn't serving red snapper, but said he didn't realize mislabeling was against the law.

"Sometimes your options are limited," he said.

Keith Deane, owner of the Gourmet House of Hong Kong, didn't know what type of fish he was getting.

"Originally we thought we were getting red snapper, but having it brought to our attention that maybe it's not red snapper, we started asking questions and discovered that, actually, we were getting sole, which is what we believe it to be now," Deane said.

The fish wasn't red snapper, but it wasn't sole either. The DNA test results showed that the restaurant was actually serving Asian catfish, which is cheaper than sole or red snapper.

"We've changed our menu to reflect, instead of red snapper, it's going to say filet of fish, because now we're not sure what it is that we have."

The last restaurant the 5 Investigates visited was the House of Tricks in Tempe. Again, the red snapper came back as grouper.

The restaurant's invoice, however, showed that the House of Tricks did order and pay for red snapper.

So what happened?

Their distributor, Santa Barbara, Calif.-based Kanaloa Seafood, sent a fax saying the fish they delivered was actually silvergray rockfish, a completely different species of fish.

5 Investigates headed to Santa Barbara to find out how the fishery could get away with selling a cheaper fish and calling it red snapper.

Tune in to CBS 5 on Tuesday night at 10 to find out what happened.