The role of the ectoparasite Caligus sp. (Copepoda: Caligidae) in the northeast Georges Bank cod-haddock ecosystem was examined. Vertical distribution of free-living Caligus elongatus adults and host-parasite relationships of juvenile CaLigus sp. and juvenile gadids are described at 2 locations with contrasting oceanographc properties, one thermally stratified and the other well-mixed. Cod Gadus morhua had both greater prevalence and number of Caligus sp. ectoparasites than did haddock Melanogran~mus aeylefinus at both locations. Preferred sites of attachment on the host also differed. While no direct evidence of reduced fish condition as a function of parasite infestation was found, circumstantial evidence is offered in support of the hypothesis that Caligus sp. ectoparasitism is a source of mortality for young haddock. Free-living C. elongatus were demonstrated to b e a significant component of fishes' diet, particularly for cod at the stratified site where zooplankton were less abundant.