At least four alligators have been seen in northern US cities in recent weeks, far from their usual southern habitat.
Analysts said the beasts were abandoned pets, not global warming refugees.
"People buy them as pets and then they get too big and at some point they decide they just can't deal with it," said Kent Vliet, a University of Florida alligator expert.
The reptiles have turned up in a Chicago river, a New York City street and a Massachusetts town this month.
On Sunday, an off-duty firefighter discovered a three-foot (0.9m) alligator sauntering across an avenue in Brockton, Massachusetts. It was wearing a spiked collar and dragging a broken leash.
In the Queens borough of New York City, also on Sunday, a two-foot reptile was found under a parked car. City officials, who described the animal as "harmless", impounded it at a local police station.
This month, a volunteer animal rescuer known only as "Alligator Bob" pulled two of the reptiles out of the murky Chicago River, which runs through the heart of America's third largest city.
"It's physically exhausting but enjoyable," Alligator Bob, who refuses to divulge his last name, told the Chicago Tribune newspaper on Wednesday after pulling a three-footer out of the river into his canoe.
Alligators are more usually found in south-eastern US states, including Florida, Louisiana and parts of Alabama and Mississippi.