Technical Abstract: The alfalfa leafcutting bee (Megachile rotundata (Fabr.)) is a solitary bee that is managed on a large scale for pollination of alfalfa seed crops. The bees nest in holes drilled in wood or polystyrene blocks, and their larvae are highly prone to a fungal disease called chalkbrood. The most prevalent form of chalkbrood is caused by Ascosphaera aggregata, an Ascomycete that is difficult to culture. Hyphae will grow on standard fungal media, but spore germination is difficult to achieve and highly variable. We found that lipids enhance germination, and compared the effects of internal lipids derived from the bee host with oils from various plants. Plant and bee lipids increased germination from 50% (without oil) to 75-85% (with oil). Percent germination was significantly greater in the presences of lipids, but germination was not significantly different when different oils were used. A. aggregata spores oriented along the lipid/aqueous interface in the broth in a polar fashion, with swelling and germ tube formation always occurring into the aqueous portion of the broth. The other half of the spore tended to attach to a lipid droplet, where it remained, without swelling, during germ tube formation. A. aggregata is a host-specific, obligate pathogen. It is possible that it has developed a specialization of utilizing lipids as a carbon source because larvae of this bee have an unusually high lipid content for an insect, storing at least twice as much lipid as most other insects. Further research is needed to determine where the spores are encountering lipids when germinating in the host gut, where germination normally occurs.