A mutualistic relationship exists with the gray whale, Eschrichtius robustus, and two species of Whale-Lice (Amphipoda: Cyamidae) because of the feeding regime of the cyamids. Cyamus scammoni Dall, 1872 and Cyamus ceti (Linnaeus 1758) apparently feed on the irritated epidermis of the gray whale surrounding the embedded, species-specific, ectocommensal barnacle Cryptolepas rhachianecti Dall, 1872 (Balanomorpha: Coronulidae). Feeding by the cyamids on the supporting skin may undermine the barnacle and ultimately cause it to become detached. The feeding area under and adjacent to the barnacle undergoes a color change from the original gray-black to white resulting in the white, circular scars characteristic to the gray whale. A third species of Cyamid, Cyamus kessleri Brandt, 1872 also occurs on the gray whale, but its role has yet to be determined.