It was shown that the ability of three different strains ofPenicillium expansum(NRRL 2034, NRRL 6069, and CBS 481.84) to grow and to produce patulin in pears (cv. Spadona) and apples (cv. Starking) varied under the different temperatures tested (0, 3, 6, 17 and 25°C). Strain NRRL 2034 did not produce patulin at 0 or 25°C while the two other strains produced the toxin at all temperatures, the maximum production occurring at 25°C (for pears) and 17°C(for apples). No significant differences in pathogenicity, as determined by lesion diameter, was recorded among the strains. Patulin production was totally inhibited when the fungi were grown in apples stored under 3% CO2/2% O2(25ˆC). The weight of infected tissue in apples contaminated with any of the strains and stored at that modified atmosphere was 70% that of the control. Under 3% CO2/10% O2or 3% CO2/20% O2, strain NRRL 2034 did not produce patulin while the two other strains produced the toxin in different amounts.