Ecotypes are groups within a species with different ecological adaptations than their conspecifics. Eastern North Pacific (ENP) killer whale (Orcinus orca) ecotypes differ in their diet, behavior, and morphology, but the same is not known for this species in the eastern Canadian Arctic (ECA) and Northwest Atlantic (NWA). Using compound-specific stable isotope analysis (CSIA) of amino acids (AAs), we compared [delta].sup.15 N patterns of the primary trophic and source AA pair, glutamic acid/glutamine (Glx) and phenylalanine (Phe), in dentine collagen of (1) sympatric ENP killer whale ecotypes with well-characterized diet differences and (2) ECA/NWA killer whales with unknown diets. [delta].sup.15 N.sub.Glx-Phe was significantly higher in the ENP fish-eating (FE) than mammal-eating (ME) ecotype (19.2 ± 0.4% vs. 13.5 ± 0.7%, respectively). Similar bimodal variation in [delta].sup.15 N.sub.Glx-Phe indicated analogous dietary divisions among ECA/NWA killer whales, with two killer whales having higher [delta].sup.15 N.sub.Glx-Phe (16.5 ± 0.0%) than the others (13.5 ± 0.6%). Inferences of dietary divisions between these killer whales were supported by parallel differences in threonine [delta].sup.15 N (-33.5 ± 1.6% and -40.4 ± 1.1%, respectively), given the negative correlation between [delta].sup.15 N.sub.Thr and TP across a range of marine consumers. CSIA-AA results for ECA/NWA whales, coupled with differences in tooth wear (a correlate for diet), are consistent with ecotype characteristics reported in ENP and other killer whale populations, thus adding to documented ecological divergence in this species worldwide.