Nodular lesions containing Hepatozoon sp. schizonts or merozoite-gametocytes were found in the tissues of 67 (96%) of 70 wild-caught martens (Martes melampus) examined in Gifu, Japan, 1991 and 1992. The heart was the most commonly parasitized organ (96%), followed by the perirenal adipose tissue (36%); the diaphragm, mesentery, tongue, omentum and perisplenic adipose tissue generally had a prevalence of 10 to 15%. In the heart, two types of nodular lesions were differentiated based on developmental stages: nodules containing schizonts and nodules consisting of an accumulation of phagocytes containing merozoites or gamonts. Under electron microscopy, mature schizonts contained membrane-bound merozoites with a single nucleus and small scattered electron-dense cytoplasmic granules in the schizont nodules; the merozoites and gamonts were engulfed in a phagosome-like vacuole of phagocytes with the nucleus compressed to one side due to the parasite in the merozoite-gamont nodule.