Previous marine biology studies found that the concentration of heavy metals in some parasites of fish such as acanthocephalans can be a proper bioindicator. Therefore, we attempted to measure five heavy metal concentrations in the tissues of the fish Gasterosteus aculeatus (G. aculeatus) and its acanthocephalan parasites, Corynosoma caspicum (C. caspicum) from the Southern Caspian Sea, northern Iran. G. aculeatus (three-spined stickleback) was collected from the south of the Caspian Sea, Mazandaran Province, northern Iran. After tissue preparation, the heavy metal concentrations in fishes and acanthocephalans were obtained using the tissue dissolution technique and an atomic absorption spectrophotometer. The concentrations of chromium (Cr), cadmium (Cd), lead (Pb), zinc (Zn), and copper (Cu) in the skin, liver, muscle, and intestine tissues of the fish and its parasites, C. caspicum, were measured and compared. Eighty (32%) of 250 collected fish were infected by at least one acanthocephalan parasite. The Cr indicated the highest concentration (5.329±3.275) of the heavy metals in acanthocephalan, even more than the skin, liver, and muscle of infected fishes. Cd had the lowest concentration (0.0333±0.0075) of heavy metals in acanthocephalan, but it was still higher than the concentration in the infected fishes' skin, liver, muscle, and intestine tissues. Our findings indicated that C. caspicum parasites can be considered extremely sensitive early-alert bioindicators, particularly in sensitive and under-threat environments with low pollution levels.