Abnormal swimming and major mortalities of fish were observed by a fish farmer in his pond in which he was rearing common carp (Cyprinus carpio) in Samsun, Turkey. A total of 118 common carps of different size and weight were transferred to the laboratory and examined for ecto- and endoparasites. Thus, a heavy infestation with the fish louse, Argulus foliaceus, was found, with the ectoparasites firmly attached to the skin and fins of all the (118) carps, during this parasitological examination (100% prevalence). Up to 800-1000 lice were counted on a single fish and most of these were detected as microscopic juvenile stages. Major fish mortalities or large numbers of moribund fishes due to this parasite were seen throughout the pond. Furthermore, no endoparasites were found in the carps examined. In the pathological investigation, hyperaemia, especially on the pectoral and pelvic fin bases and in the abdominal area, was macroscopically observed.