Abstract
Myxidium zealandicum Hine, 1975 was recorded from several tissues in New Zealand freshwater eels. The gills were the most frequent site of infection, but spores also matured on the gill arch, in the skin, and occasionally in the urinary and swim bladders. Thus M. zealandicum is a histozoic species that may have evolved from a coelozoic form. Variation in spore size (8.0–11.5 jam) was found in relation to site of infection; variation in the number and arrangement of valve striations was also observed. Deposition of fibro‐blasts occurred at all sites of sporogony, except among the goblet cells of the epidermis. The small size (6.0–10.5μm) of unstriated spores occurring with normal spores in the gills of eels from Lake Otomangakau appeared to be related to the size of the cysts in which they occurred, and to the host Anguilla dieffenbachii.