The American Bullfrog (Rana catesbeiana) is an aquatic frog, a member of the family Ranidae, or "true frogs", native to much of North America. This is a frog of larger, permanent water bodies, swamps, ponds, lakes, where it is usually found along the water's edge . On rainy nights, bullfrogs along with many other amphibians, go overland and may be seen in numbers on country roads.

American bullfrogs live longer in warm weather. They have been widely introduced across North America (see range map). The original, naturally determined range did not include far western regions where it is found today.

The adult bullfrog skeleton is representative of tetrapod vertebrates, comprising an axial skeleton (skull and vertebrae) and an appendicular skeleton (pectoral girdle and forelimbs, pelvic girdle and hindlimbs). Ranids, however, lack ribs. The pronounced pair of dorsal humps in the back of ranid frogs are the ends of the pelvic ilia, homologues of the human hips.

The bullfrog skull is highly fenestrated. The orbits open ventrally through the roof of the mouth to accommodate eye retraction during locomotion and swallowing. The skull bears a continuous row of tiny teeth on the maxilla and premaxilla and a pair of small vomerine teeth on the palate. The mandible is toothless.

The bullfrog nervous system consists of a brain, spinal cord, and peripheral nerves including cranial nerves, spinal nerves, and sympathetic nerves serving organs such as the heart, gastrointestinal tract, kidneys, gonads.

Ranid frogs absorb oxygen and eliminate carbon dioxide through their moist skin, the lining of the mouth, and the lungs. When in the air, as opposed to underwater, frogs continuously elevate and lower the floor of the mouth, which serves to ventilate the mouth, or buccal cavity, and exchange gases through the richly vascularized lining of the mouth. Periodically, the regular rhythmic pumping of the floor of the mouth is interrupted by a deeper lowering of the throat at the extreme of which the glottis opens and the throat muscles contract vigorously to force air from the mouth into the lungs—the nostrils are closed off. This lung ventilation may be performed several times after which the shallow buccal ventilation resumes. Lacking ribs, frogs must supply the pressure to force air into their lungs, whereas mammals can enlarge the cavity surrounded by the rigid rib cage and allow the atmosphere to supply the pressure.

Fertilization is external in ranid frogs. In the mating grasp, or amplexus, the male rides on top of the female, grasping her with his forelimbs posterior to her forelimbs. The female bullfrog deposits her eggs in the water and the male simultaneously releases sperm.

breeding begins in late spring or early summer. Males defend and call from territories, attracting females into a territory to mate. The call is reminiscent of the roar of a bull, hence the frog's common name. A female may produce up to 20,000 eggs in one clutch.

Eggs hatch in 3–5 days. Time to metamorphosis ranges from a few months in the southern to 3 years in the northern parts of the geographic range.

Maximum lifespan in the wild is estimated at 8–10 years, but one captive lived almost 16 years.

Stomach content studies going back to 1913 suggest the bullfrog preys on any animal it can overpower and stuff down its throat. Bullfrog stomachs have been found to contain rodents, small turtles, snakes, frogs including bullfrogs, birds, even a bat, as well as the many invertebrates which are the usual food of ranid frogs . These studies furthermore reveal the bullfrog's diet to be unique among North American Rana in the inclusion of large percentages of aquatic animals, e.g., fish, tadpoles, Planorbid snails, Dytiscid beetles. The specialized ability of bullfrogs to capture submerged and large strong prey comprises a pronounced biting motor pattern that follows up on the initial and typical ranid tongue strike . Adaptation to target image displacement due to light refraction at the water-air interface consists of the bullfrog's application of tongue surface comparatively posterior to the perceived location of the prey target . The comparative ability of bullfrogs to capture submerged prey, compared to that of the green frog, leopard frog, and wood frog (R. clamitans, R. pipiens, R. sylvatica, respectively) was also demonstrated in laboratory experiments.

The bullfrog, like many other ranid frogs, is an ambush predator. At a chosen site, the visually vigilant frog waits relatively motionless until prey appears and elicits its feeding behavior.

Prey motion elicits feeding behavior. First, if necessary, the frog performs a single orienting bodily rotation ending with the frog aimed towards the prey. This is followed by approaching leaps, if necessary. Once within striking distance, the bullfrog emits its feeding strike, which consists of a ballistic (eyes closed as during all leaps) lunge that ends with the mouth opening, extension of the fleshy and mucous-coated tongue upon the prey, often engulfing it, while the jaws continue their forward travel to close (bite) in close proximity to the prey's original location just as the tongue is retracted back into the mouth, prey attached. Large prey that do not travel entirely into the mouth are literally stuffed in with the forearms. In laboratory observations, bullfrogs taking mice usually dove underwater with prey in mouth, apparently with the advantageous result of altering the mouse's defense from counterattack to struggling for air. The tiny teeth of bullfrogs are useful only in grasping. Asphyxiation is the most likely cause of death of endothermic (warm-blooded) bullfrog prey.

While occasionally kept as pets, the American Bullfrog provides a minor food source, especially in the Southern United States and in some areas of the Midwestern United States. In a few locations they are commercially cultured in ponds, but the traditional way of hunting them is to paddle or pole silently by canoe or flatboat in streams or swamps at night; when the frog call is heard, a light is shined on the frog to temporarily inhibit it. The frog will not jump into deeper water as long as movement is slow and steady. When close enough, the frog is gigged and brought into the boat. In some states, breaking the skin while catching them is illegal and either grasping gigs or hand capture are used. The only parts eaten are the rear legs, which resemble small chicken drumsticks and, sometimes, the backs, which are usually fried for consumption.

The American Bullfrog is also used as a specimen for dissection in many schools across the world.