Abstract Fossil mollusks associated with eight Oligocene whales from the Makah and Pysht Formations on the northwestern part of the Olympic Peninsula, Washington, suggest that whale carcass sulfide production supported small numbers of some chemosymbiotic invertebrates as early as 30 million years ago. Thyasirid and modiolid bivalves usually dominate these very localized molluscan assemblages; lucinid and nuculanid bivalves, scaphandrid, naticid, and buccinid gastropods are rarely present; brachiopods were found once. These fossils include, tentatively, the first fossil record for the bivalve genus Idasola, and the first record of the bivalve Thyasira peruviana Olsson, outside of probable cold-seep deposits in South America. Strata surrounding the fossil whales contain low diversity megafaunas that include rare deep-water gastropods and bivalves, large isopods, and localized cold-seep communities. The whale-fall assemblages differ significantly because of the presence of Idasola? sp. and Thyasira peruviana? and the absence of vesicomyid bivalves. Vertebrate carcasses have probably not contributed significantly to the dispersal of cold-seep and hydrothermal vent invertebrates. Seep/vent communities were well established much earlier than the evolution of cetaceans, and seep/vent invertebrates have not been found with carcasses of other large vertebrates.