The mitochondrial DNA restriction fragment patterns of killer whales (Orcinus orca) were investigated in order to determine the level of genetic differentiation that exists between killer whales from various geographic locations. Twenty one killer whales were examined, seventeen of which were captive killer whales that originated from the North Atlantic and Northeast Pacific Oceans. Two were captive-born animals and two were killer whales that stranded along the Northeast Pacific coast. DNA was extracted from blood and/or tissue samples, cleaved with a variety of restriction endonucleases and the DNA fragments were separated by horizontal agarose gel electrophoresis. The DNA was then transferred to nylon membranes and the killer whale mitochondrial DNA was visualized by hybridization to the complete mitochondrial DNA genome of Commerson's dolphin (Cephalorhynchus commersonii). The resultant restriction fragment patterns were analyzed to determine whether mitochondrial DNA variation was present between killer whales from different geographic regions or between communities and pods of killer whales from the same geographic location.