Sixty specimens of Paralichthys isosceles captured in the southeastern Brazilian coast were investigated for helminths from October 2006 to March 2008. One thousand eight hundred and twenty larvae (third and fourth stages) of Hysterothylacium sp. were recovered from the abdominal cavity, free or inserted in the mesentery, intestine, stomach, stomach wall, liver, spleen, ovaries, and serosas of heart, kidneys, ovaries and abdominal musculature. Gross lesions appeared as nodules in the stomach mucosa and intestinal serosa. Histological examination of these nodules revealed sections of Hysterothylacium sp. larvae inserted in the abdominal musculature, in the submucosa, muscular and serosa layers of the stomach and intestine. Associated granulomas consisted of fibrous connective tissue, macrophages and lymphocytes that surrounded, besides the larvae, necrotic material and tunnels with acidophilic lining, sometimes devoid of inflammatory reaction. The pathological alterations as well as the occurrence of Hysterothylacium sp. are reported for the first time in this host. The parasitic indexes refer to a prevalence of 100%, mean intensity of 30.3, mean abundance of 32 and range of intensity variation from 1 to 596.