The distribution of the parasitic copepod, Salmincola californiensis, on two size-groups (fry, fork length 3.2–5.8 cm; juveniles, fork length 10.2–27.0) of the sockeye salmon, Oncorhynchus nerka, is described. The existence of preferred sites for both groups is established. The distribution on the smaller fish differs from that on the larger in that on the former the copepods are most abundant in the region of pectoral and pelvic fins, whereas on the latter they are by far most common in the branchial cavity. Macroscopic and microscopic mechanical damage to the fish tissues, resulting from the presence and activity of the copepod, comprises injuries to gills, skin, muscle, and even bone. The "burrowing phenomenon" (failure on the part of the copepod to cease excavation of a completed cavity of implantation, resulting in perforation of body wall and penetration of the viscera) is reported upon for the first time.