We have described the morphology and life cycle ofApolocystis sp., parasite in the seminal vesicles of the earthwormOctolasium transpadanum(Rosa). The life cycle was investigated by light microscopy but only the mature trophozoite stage by electron microscopy. The trophozoite is spherical, active, rotating quickly. Its ultrastructure conforms broadly to that of other gregarines. The pellicle is composed of 3 parallel unit membranes and forms a number of epicitary folds arranged as a labyrinth. These folds are not stable structures and may appear anywhere on the trophozoite surface. Micropores concerned with feeding, are randomly distributed on the pellicle. Many haptonemata start from the surface and pass through the barrier of the amoebocytes that always surround the trophozoite. A precise limit between the ectoplasm and the endoplasm does not exist; mitochondria, dense bodies and other inclusions are found within the top of the folds. Mitochondria are located superficially. Microtubules enter into the haptonemata and are found beneath the pellicle where they constitute a cytoskeleton. The cytoplasm has a poorly developed reticulum, but is filled with free ribosomes, with typical paraglycogen granules and numerous other inclusions. The nucleus has a homogeneous matrix and a compact spherical nucleolus. The role of the fine morphology in the taxonomy of Monocistidae gregarines is discussed.