The effect of high hydrostatic pressure treatment (with pressures of up to 700 MPa) on Talaromyces macrosporus ascospores was investigated. At 20degreesC, pressures of greater than or equal to200 MPa induced the activation and germination of dormant ascospores, as indicated by increased colony counts for ascospore suspensions after pressure treatment and the appearance of germination. vesicles and tubes.. Pressures of >400 MPa additionally sensitized the ascospores to subsequent heat treatment. At pressures of >500 MPa, activation occurred in a few minutes but was followed by inactivation With longer exposure. However, even with the most extreme pressure treatment, a fraction of the ascospore population appeared to resist both activation and inactivation, and the maximal achievable reduction of ascospores was on the order of 3.0 log(10) units. Pressure-induced ascospore activation at 400 MPa was temperature dependent, with minimum activation at 30 to 50degreesC and greater than or equal to10-fold higher activation levels at 10 to 20degreesC and at 60degreesC, but it was not particularly pH dependent over a pH range of 3.0 to 6.0. Pressure inactivation at 600 MPa, in contrast, was pH dependent, with the inactivation level being 10-fold higher at pH 6.0 than at pH 3.0. Observation of pressure-treated and subsequently dried spores with the use of light and scanning electron microscopy revealed a collapse of the spore structure, indicating a loss of the spore wall barrier properties. Finally, pressure treatment sensitized T. macrosporus ascospores to cell wall lytic enzymes.