ABSTRACTUntil recently, pansporoblastic microsporidia that produce a variable and large number of sporoblasts from a sporont have been included in a single genus, namelyPleistophoraGurley, 1893. Ultrastructural studies have been used to determine whether the resemblance of these species is fundamental or superficial. The results indicated that the multisporous pansporoblastic forms belong to at least three genera and, thus, thatPleistophorais a “composite genus.” The term pansporoblast was originally used for stages in myxosporidian development. The term sporophorous vesicle adopted from Gurley is suggested for the spore‐containing vesicle in the Microspora. Three species were studied:Pleistophora typicalis, the type‐species;Pleistophora culicis, for which a new genusVavraiahas already been proposed; andPleistophora simulii. P. typicalisandV. culicishave isolated nuclei throughout their development, and the sporophorous vesicle wall enveloping the sporoblasts is derived from amorphous secretions laid down during merogony external to the plasmalemma.PleistophoraandVavraiaare differentiated principally in terms of the structure of the sporophorous vesicle wall and mode of division of the sporogonial plasmodium. The nuclei of young sporonts ofP. simuliiare in diplokaryon arrangement and undergo meiosis to give haploid nuclei in the sporoblasts. The sporophorous vesicle wall is membranoid and is laid down external to the plasmalemma at the onset of sporogony. A new genus,Polydispyrenian. g., is suggested for this species, the affinities of which are closer to the dimorphic species of microsporidia than toPleistophoraorVavraia. The terms “merontogenetic sporophorous vesicle” and “sporontogenetic sporophorous vesicle” are used to distinguish between