The croak gave it away.

On a foray into the wilds of Staten Island in 2009, Jeremy A. Feinberg, a doctoral candidate in ecology and evolution at Rutgers University, heard something strange as he listened for the distinctive mating call of the southern leopard frog — usually a repetitive chuckle. But this was a single cluck.

“I started hearing these calls, and I realized they were really distinct,” Mr. Feinberg said.

Three years later, Mr. Feinberg and four other scientists who joined him in multiple field and laboratory studies, are finally comfortable making their declaration: a new species of leopard frog — as yet unnamed — has been identified in New York City and a number of surrounding counties.

The find is surprising on a number of fronts, not least of which is that the new frog was hiding in plain sight in one of the most populated centers in the world. (Most new species are found in remote areas.) And it illustrates the power of genetic testing in parsing more finely those animals that may be nearly identical in appearance, but are, in fact, of different species.

There are more than a dozen leopard frogs, ranging from Canada to Central America. Medium in size, with dark spots on a tan, olive or green background, they gravitate toward grassy meadows and breed in ponds or pools. The researchers say that the new frog species was confused for a long time with the southern leopard frog, which it closely resembles.