The genus Nosema Naegeli, 1857, is characterized largely by reference to its type-species Nosema bombycis Naegeli. As early as 1905, Pérez gave a rather good characterization of this species, introducing the term “monosporous.” Unfortunately, this term soon acquired a generally accepted change of meaning that caused a half-century of confusion. Modern concepts clarify and reaffirm the limited definition given by Pérez and include also other essential characters. Some of these other characters, applicable to the genus as well as to the type-species, are: apansporoblastic, monomorphic, disporous, diplocaryotic. Available data on species of Nosema reported in the literature were compared with the modern concepts of the generic characters and the genus Nosema was found to be quite heterogeneous. Consequently, 23 species were transferred to the collective group Microsporidium. Remaining in the genus were 137 named species and at least 27 unnamed ones. Many of these were very poorly described and an appreciable number of them probably are not congeneric with N. bombycis.