Technical Abstract: Nosemosis (Nosema disease) is one of the most serious and prevalent adult honey bee diseases worldwide. For years, Nosema apis was thought to be the only microsporidia infecting domestic bee colonies. However, recently it was discovered that N. ceranae could jump from Asian honey bees (Apis cerana) to European honey bees (Apis mellifera) that are widely used for crop pollination. The studies were conducted to investigate N. ceranae infection of A. mellifera in the U.S. and Asia. The data presented in the studies demonstrated that N. ceranae infection is widespread in the U.S., China and Australia and that infection with N. ceranae was more common than infection with N. apis in European honey bees. However, the N. ceranae in Australia may have a relatively recent introduction compared to other regions of the world. While comparative structural models of ribosomal RNAs indicate that ribosomal RNAs of N. ceranae and N. apis are well conserved, significant differences in morphology, tissue tropism, and phylogeny exist between species of N. ceranae and N. apis. The finding about the prevalence of N. ceranae in the U.S. and Asian bee populations in conjunction with findings in other parts of the world invites further research of the evolutionary history of N. ceranae infection in European honey bees.