Two natural populations of Artemia sp. (SW and SE coast of Spain) were infected by microsporidia. The cytology and the life cycle of the most important species, Nosema artemiae (CODREANU, 1957) SPRAGUE, 1977, is described here with focus on the ultrastructure. The parasite developed in the musculature, haemocytes and hypoderm. All stages had nuclei coupled as diplokarya and the reproduction was by binary fission. Merozoites were frequently arranged in chains. The developing sporont wall, passed through a series of stages, where the electron-dense material secreted on the plasma membrane was rearranged: initially seen as tubulus-like structures, then as parallel stands, and finally as a confluent uniform layer. Spores were ovoid measuring 2.8-3.5 x 3.9-5.11 mum in living condition. The spore wall had the normal three components: plasma membrane, a structure-less endospore, and an exospore composed of two layers of different electron-density. The polar filament was either isofilar, with 11-13, or lightly anisofilar with 13-16 coils. The polaroplast had two regions with lamellar compartments. The anterior lamellae were more closely packed.
