Polysporoplasma sparis infection was studied in gilthead sea bream from different mariculture systems of the Spanish coasts. Culture conditions influenced the infection dynamics, as the parasite appeared only in semi-intensive cultures and was not found in intensive closed systems nor in open ones. No clear seasonal pattern was observed. No fish weighing less than 51 g was found parasitized in any group. A statistically significant dependence between infection prevalence and host weight was observed in some growing stocks. Light and transmission electron microscope observations revealed serious damage in the trunk kidney. Glomerular disease was provoked by the progressive occupation of the glomerular capillaries by P. sparis spores. Tubular epithelial cells were also affected. Inflammatory responses appeared towards the end of the infection, and consisted mainly of melanomacrophages and eosinophils. Rodlet cells were common close to infected capillaries and debris of rodlet sacs formed a belt encircling capillary vessels. Cytochemistry demonstrated the lipidic nature of these sacs and the glycogen and glycoprotein composition of the cytoplasmic granules of rodlet cells.