A new microsporidian parasite of a freshwater cladoceran from southern Sweden is described using light and electron microscopical methods. Development comprises 2 merogonial sequences, the first resulting in a cluster of 8 merozoites, the second in a chain of 4 merozoites. Each secondary merozoite develops into a sporont which divides into 2 sporoblasts, each of which develops into a spore. The spores are broadly oval and in fresh smears measure about 6 μm in length, with a single nucleus and a posterosome. The polar filament is about 40 μm long, of even thickness throughout, and appears as 15—18 coils in a single layer. The anchoring disc is small and the polaroplast is composed of 2 lamellar parts. Outside the plasma membrane of the sporont a 5-layered, electron-dense substance is produced, which further differentiates into endo- and exospore, an electron-dense substance occurring patchily on the exospore and a pansporoblast membrane. During development the sporoblasts and the young spores are connected by a dense substance. Mature spores appear single or paired. The pansporoblast membrane is composed of 2 structurally different layers, namely a thin outer, single membrane and an inner layer composed of tubular structures. It is connected to the spore coat by patches of the dense substance. The new microsporidium is considered to belong to a new genus of the family Telomyxidae, and its systematic relationship with this and the related family Tuzetiidae is discussed. A survey of microsporidia from Cladocera is included.