The endangered beluga whales (Delphinapterus leucas) in Cook Inlet are an important cultural and subsistence resource to coastal Alaska Natives and to the Cook Inlet ecosystem as a top-level predator. To better understand the past feeding ecology of beluga whales from Cook Inlet, we sampled bone (n=20) and annual growth layer groups (GLG; n=412) in teeth (n=26) for isotopic analyses. Both the bone and GLG data showed a general decrease in their δ13C (~2‰) and δ15N (~1‰) values between 1962 and 2007 indicating a change in trophic and prey source shift. To better understand the shift in prey source we analyzed strontium isotope composition (87Sr/86Sr) in GLGs for a sub-set of belugas and found a trend from marine towards a more freshwater signature. These results indicate a change in feeding ecology and a shift towards more freshwater influenced habitats both of which are consistent with survey data showing the summer range retracting into the upper reaches of Cook Inlet.