Recent research suggests revision of the traditional naming of ratsnakes is needed. Before 2001, all Ontario populations were called the black ratsnake (Elaphe obsoleta), but subsequent studies identified several distinct species that were formerly confused under this name.

The biggest change is the reassignment of North American ratsnakes to a different genus. The North American ratsnakes are now assigned to the genus Pantherophis, and the genus Elaphe is now restricted to the Eurasian species.

Furthermore, the populations in southwestern Ontario that used to be known as black ratsnakes are now assigned to the gray ratsnake—also called midland ratsnake—(Pantherophis spiloides), and the eastern Ontario ones are recognized as hybrids between this species and the eastern ratsnake (Pantherophis alleghaniensis).

Even more recently, an alternate view has proposed combining all the ratsnakes and the bullsnakes together in the genus Pituophis. Further study will clarify classification.

The species found in Ontario may be the largest snakes found in Canada. Individuals longer than 1.5 m (5 ft.) are not uncommon, and even-longer ones have been recorded. The young are about 28 to 33 cm long (11 to 13 in.) at birth.

Hatchlings are pale grey with black blotches along the back. This pattern fades quickly, and the snake will attain adult colouration in about two or three years, becoming black with little trace of the pattern. Their belly is white with black checkerboard markings and they have a white chin.

This ratsnake feeds primarily on mice, rats, and some bird eggs and young birds. Not venomous, it constricts its prey within the tight coils of its body. Adults often hunt for mice around rural buildings such as barns, sheds, or cottages. The young will feed on small lizards, young mice and, occasionally, on small frogs such as treefrogs.

An excellent and frequent climber, it sometimes shelters in cavities high up in hollow trees. It generally inhabits uplands away from water. In Canada, it occurs in small numbers in southwestern Ontario just north of Lake Erie but is more common at the eastern end of Lake Ontario in the rocky, mostly non-agricultural terrain of the Rideau Lakes region, from Kingston to south of Smiths Falls.