In the last century, nosemosis caused by Nosema apis is traditionally considered as a low-prevalence disease of Apis mellifera, even though it occurs worldwide. Colonies affected by N. apis display low levels of infection during summer, a small peak in autumn and usually a slow rise during winter. However, nosemosis due to Nosema ceranae is considered as an emergent illness that is posing a major threat to the health of individual honey bees and whole bee colonies. The symptoms of infection by these two pathogens are very different, as are the virulence, spread and pathogenicity. We have carried out experiments in artificially infected worker honey bees maintained in the laboratory at two different temperatures. Both microsporidia developed as expected for up to 4 days after infection at 33.0°C, but when maintained for 5 or 7 days at 37.2°C, only N. ceranae completed its life cycle in infected honey bees, while the development of N. apis was inhibited. This and other published data suggest that N. ceranae is eurythermal whereas N. apis is stenothermal. The higher temperature tolerance recorded may be related to the higher prevalence of N. ceranae reported worldwide.