Epithelial hyperplasia was the most distinctive pathological change in gill filaments of striped bass, Morone saxatilis (Walbaum), from the lower Chesapeake Bay infested by Ergasilus labracis Krøyer. This tissue reaction initiated in the filament area most adjacent to the mouth of the attached parasite. Interruption of parasite egg sac production and eventual dislodgement associated with swelling of the gill filaments occurred unilaterally on gill arches of either side of the branchial basket. The largest number of parasites in all fish was found on the first outer hemibranch while abundance on successive hemibranches decreased in rank order. Copepods on the inner hemibranch, however, were least numerous on the first inner hemibranch and more numerous on the fourth inner hemibranch. On the whole, the number of parasites attached to the outer hemibranch outnumbered the number of parasites established on the inner hemibranchs. This differential settlement was most evident in low to moderate infections and became less apparent in heavy infections. Newly settled copepods showed highest abundance on the third arch and least abundance on the first and fourth arches. The causes for this differential distribution on the gill arches are discussed.