Currently, three species of the eukaryotic obligatory intracellular microsporidian parasites of the genus Nosema have been identified in bees, namely Nosema bambi infecting bumblebees (Bombus spp.) and Nosema apis and Nosema ceranae found in honeybees (Apis spp.). Given difficulties in identifying them Cj visually, I developed molecular genetic markers for these species and used them to understand their distribution and modes of transmission. I developed a PCR based method for the specific and sensitive diagnosis of N. bombi in bumblebees through amplification of partial ribosomal RNA gene fragments. A survey of N. bambi in European bumblebees allowed me to reveal that the parasite is common and widespread. Furthermore, partial rRNA sequences of microsporidians of North American bumblebees were very similar to European ones, suggesting that N. bambi has a Holarctic distribution. I conducted an , experiment to test for horizontal transmission of N. bambi via artificially contaminated flowers, and I analysed secondary hosts and wild flowers using molecular genetic ·tools for the presence of N. bombi. I was unable to demonstrate horizontal transmission, but suggest that the phoretic mite Parasitellus fucarum may act as a secondary host of N. bambi. In a further study, I found bumblebee ovaries to contain N. bombi, which hints at vertical transmission. Using in situ hybridisation of bumblebee ovaries, I was unable unequivocally to locate N. bambi. I then developed a method to differentiate between N. apis and N. ceranae based on PCR-RFLPs of partiat' small subunit rRNA and employed it to analyse microsporidian isolates of the Western honeybee Apis mellifera from across the world. From the data generated, I conclude that N. ceranae most likely jumped host from the Asian honeybee A. cerana to A. mellifera, probably within the last decade;N. ceranae is now found across most of the world. Using a temporal sequence of Nasema isolates from Finland from 1986-2006, I found that N. ceranae has been present in Europe since at least 1998.