Abstract Gorodkowa agar (GA), Fowell's acetate agar (FAA), Kleyn's acetate agar (KAA), McClary's acetate agar (MAA), grape juice agar (GJA), apple juice agar (AJA) and vegetable juice agar (VJA) were evaluated for their efficacy to promote ascospore formation by three strains of Neosartorya fischeri. All three strains produced abundant mature ascospores when grown on FAA, GJA and AJA, but not on the other test media. Ascospores produced on GJA developed greater heat resistance with time when suspended in commercial apple juice (AJ), grape juice (GJ) and 0·1 M potassium phosphate buffer (PB) during heat treatment. From 10 to 13 days of incubation at 30°C, decimal reduction times at 80°C ( D 80°C values) for ascospores increased from 27·0 to 66·7 min in AJ, 28·5 to 33·3 min in GJ and 18·2 to 50·0 min in PB. The rate of development of heat resistance was dependent upon the incubation temperature at which ascospores were produced. After treatment at 70°C for 60 min, survival percentages for 21-day-old ascospores were 0·32, 8·13, 54·95 and 53·70% of ascospores produced at 18, 21, 25 and 30°C, respectively. At 42 days, 0·23 and 38·02% of ascospores produced at 18 and 21°C, respectively survived 85°C for 30 min; this treatment did not inactivate 42-day-old ascospores produced at 25 and 30°C. During a 114-day test period, ascospores produced at all incubation temperatures became dormant to some extent and required heat activation to facilitate germination.