The composition of the metazoan parasite communities within 2 fishing zones along the Chilean coast were compared to determine the population structure of the jack mackerel Trachurus symmetricus murphyi (Nichols) in these 2 geographical locations. More than 7,780 parasites belonging to 15 taxa were collected between 1990 and 1996 in 71 samples comprised of 3,946 hosts. The same taxa were found in both fishing zones. However, jack mackerel from northern Chile had a higher abundance of cymothoid isopods, Ceratothoa spp., whereas those from southern Chile had more Rhadinorhynchus trachuri, Hysterothylacium sp. larvae, and Anisakis type I larvae. Results were similar at all community levels because all parasites had low prevalence and infracommunities had low total abundance and richness. Analyses at the component community level may be more adequate for stock identification when infracommunities are simple. There were significant differences in composition of parasite communities between years in each fishing zone, presumably as a result of the increase in offshore catches since 1994. These results reinforce the hypothesis that more than 1 ecological stock of the jack mackerel exists in the southeastern Pacific and contradict the current assumption of a single stock in the management of this heavily exploited fish species.