Ascospores (10 5/ml) of Neosartorya fischeri var. glaber (FRR 1833) harvested from grape juice agar after 11 d of incubation at 30°C were inactivated by treatment at 82° for 60 min; 25-day-old ascospores survived this heat treatment. To determine if differences in ultrastructure and chemical composition might be related to differences in heat-resistance, ultrastructure as well as protein, lipid, sugar and polyol contents of 11-day-old (heatsensitive) and 25-day-old (heat resistant) ascospores were compared. SEM revealed little difference in external appearance, whereas TEM revealed some internal differences between 11- and 25-day-old ascospores, particularly in the region of germ-tube initiation. Significant qualitative differences in protein content between heat-sensitive and heat-resistant ascospores were observed. The lipid and fatty acid compositions of the two types of ascospore were similar. Heat-resistant ascospores contained higher levels of glycerol, mannitol and trehalose than heat-sensitive ascospores, suggesting that the lowering of internal water activity or membrane stabilization may partially account for increased heat resistance.