The three to five types of killer whales may be distinct enough to be considered different races, subspecies, or possibly even species. The IUCN reported in 2008, "The taxonomy of this genus is clearly in need of review, and it is likely that O. orca will be split into a number of different species or at least subspecies over the next few years. Although large variation in the ecological distinctiveness of different killer whale groups complicate simple differentiation into types, research off the west coast of Canada and the United States in the 1970s and 1980s identified the following three types:

Resident: These are the most commonly sighted of the three populations in the coastal waters of the northeast Pacific. Residents' diets consist primarily of fish and sometimes squid, and they live in complex and cohesive family groups called pods. Female residents characteristically have rounded dorsal fin tips that terminate in a sharp corner. They visit the same areas consistently. British Columbia and Washington resident populations are among the most intensively studied marine mammals. Researchers have identified and named over 300 killer whales over the past 30 years.

Transient: The diets of these whales consist almost exclusively of marine mammals. Transients generally travel in small groups, usually of two to six animals, and have less persistent family bonds than residents. Transients vocalize in less variable and less complex dialects.Female transients are characterized by more triangular and pointed dorsal fins than those of residents. The gray or white area around the dorsal fin, known as the "saddle patch", often contains some black colouring in residents. However, the saddle patches of transients are solid and uniformly gray. Transients roam widely along the coast; some individuals have been sighted in both southern Alaska and California. Transients are also referred to as Bigg's killer whale in honor of Michael Bigg. The term has become increasingly common and may eventually replace the transient label.

Offshore: A third population of killer whales in the northeast Pacific was discovered in 1988, when a humpback whale researcher observed them in open water. As their name suggests, they travel far from shore and feed primarily on schooling fish. However, because they have large, scarred and nicked dorsal fins resembling those of mammal-hunting transients, it may be that they also eat mammals and sharks.They have mostly been encountered off the west coast of Vancouver Island and near the Queen Charlotte Islands. Offshores typically congregate in groups of 20–75, with occasional sightings of larger groups of up to 200.Currently, little is known about their habits, but they are genetically distinct from residents and transients. Offshores appear to be smaller than the others, and females are characterized by dorsal fin tips that are continuously rounded.

Killer whale mother and calf extending their bodies above the water surface, from pectoral fins forward, with ice pack in background

Type C killer whales in the Ross Sea: The eye patch slants forward.

Transients and residents live in the same areas, but avoid each other.The name "transient" originated from the belief that these killer whales were outcasts from larger resident pods. Researchers later discovered transients are not born into resident pods or vice versa. The evolutionary split between the two groups is believed to have begun two million years ago. Genetic data indicate the types have not interbred in the wild for up to 10,000 years.

Other populations have not been as well studied, although specialized fish-eating and mammal-eating killer whales have been distinguished elsewhere.[38] Separate populations of fish-eating and mammal-eating killer whales have been identified around the United Kingdom. As with residents and transients, the lifestyle of these whales appears to reflect their diet; fish-eating killer whales in Alaska and Norway have resident-like social structures, while mammal-eating killer whales in Argentina and the Crozet Islands behave more like transients.

Three types have been documented in the Antarctic. Two dwarf species, named Orcinus nanus and Orcinus glacialis, were described during the 1980s by Soviet researchers, but most cetacean researchers are skeptical about their status, and linking these directly to the types described below is difficult.

Some examples of variations in killer whales

Type A looks like a "typical" killer whale, a large, black and white form with a medium-sized white eye patch, living in open water and feeding mostly on minke whales.

Type B is smaller than type A. It has a large white eye patch. Most of the dark parts of its body are medium gray instead of black, although it has a dark gray patch called a "dorsal cape" stretching back from its forehead to just behind its dorsal fin. The white areas are stained slightly yellow. It feeds mostly on seals.

Type C is the smallest type and lives in larger groups than the others. Its eye patch is distinctively slanted forwards, rather than parallel to the body axis. Like type B, it is primarily white and medium gray, with a dark gray dorsal cape and yellow-tinged patches. Its only observed prey is the Antarctic cod.

Type D was identified based on photographs of a 1955 mass stranding in New Zealand and six at-sea sightings since 2004. It is immediately recognizable by its extremely small white eye patch, shorter than usual dorsal fin, and bulbous head (similar to a pilot whale). Its geographic range appears to be circumglobal in subantarctic waters between latitudes 40°S and 60°S. And although nothing is known about the type D diet, it is suspected to include fish because groups have been photographed around longline vessels where they reportedly prey on Patagonian toothfish (Dissostichus eleginoides).

Types B and C live close to the ice pack, and diatoms in these waters may be responsible for the yellowish coloring of both types. Mitochondrial DNA sequences support the theory that these are recently diverged separate species.More recently, complete mitochondrial sequencing indicates the two Antarctic groups that eat seals and fish should be recognized as distinct species, as should the North Pacific transients, leaving the others as subspecies pending additional data.

Mammal-eating killer whales were long thought likely to be closely related to other mammal-eating killer whales from different regions, but genetic testing refuted this hypothesis.

The identified seven ecotypes in isolated ecological niches. Of three orca ecotypes in the Antarctic, one preys on minke whales, the second on seals and penguins and the third on fish. Another ecotype lives in the eastern North Atlantic, while the three Northeast Pacific ecotypes are labeled the transient, resident and offshore populations. The research supported a proposal to reclassify the Antarctic seal- and fish-eating populations and the North Pacific transients, should be recognized as distinct species, leaving the remaining ecotypes as subspecies. The first split in the orca population, between the North Pacific transients and the rest, occurred an estimated 700,000 years ago. Such a designation would enable/require that each new species be subject to separate conservation assessments.