We describe the ultrastructure of two microsporidia parasites of simuliids of the region of La Calera (2.950 m alt.), near Bogota. Polydispyrenia simulii was found infecting larvae of Simulium ignescens and Polydispyrenia norandina sp. nov., infecting a larva ofGigantodax ortizi. The two species present large number of spores aggregated within a sporophoric vesicle, characteristic ofthe family Pleistophoridae. Polydispyrenia simulii, a cosmopolitan parasite of simuliid larvae, has until now been the only known species of this genus. Two ultrastructural characters permit identification of this species: a) Presence of peculiar cylindrical structures surrounding the sporoblasts in the cavity of the sporophoric vesicle (episporontal space); in cross section these structures look like thick-walled rings: b) Consistently 5-6 coils ofthe polar filament inside the mature spore. Polydispyrenia norandina sp. nov.shows metabolic products within the episporontal space thatappear as electrodense spots and 8 colls of the polar filament, both in the sporoblasts and in the mature spores.