The unknown oligochaete host in the life-cycle of Hoferellus carassii Achmerov, 1960 was identified as a member of the family Naididae, the genus Nais (most probably Nais elinguis). The aurantiactinomyxon spore found by El-Matbouli et al. ( 1992a) is demonstrated to be the actinosporean stage of Hoferellus carassii. The development of this spore within the naidid host is described by electron microscopy, and the results compared with those of Janiszewska (1955, 1957) and Marqués (1984. 1986) for Actinosporea originally described as a separate group of parasites. Additionally the plasmodial development of H. carassii within the urinary bladder of the goldfish (Carassius auratus auratus Linnaeus, 1758) is described.