The major aims of this study were to assess inter- and intra-species variability of heat resistant moulds (HRMs), Byssochlamys fulva and Byssochlamys nivea, with regards to (i) heat resistance and (ii) effect of heat treatment intensity on subsequent outgrowth. Four-week-old ascospores were suspended in buffered glucose solution (13 degrees Brix, pH 3.5) and heat treated in a thermal cycler adjusted at 85 degrees C, 90 degrees C and 93 degrees C. Two variants of the Weibull model were fitted to the survival data and the following inactivation parameters estimated: b (inactivation rate, min(-1)), n (curve shape) and delta (the time taken for first decimal reduction, min). In addition to the assessment of heat resistance, outgrowth of Byssochlamys sp. from ascospores heated at 70 degrees C, 75 degrees C, 80 degrees C 85 degrees C and 90 degrees C for 10 min and at 93 degrees C for 30 and 70 s was determined at 22 degrees C for up to 30 days. The Baranyi and Roberts model was fitted to the growth data to estimate the radial growth rates (mu(max), mm.day(-1)) and lag times (lambda, days). Inter species variability and significant differences (p < 0.05) were observed for both inactivation and growth estimated parameters among B. fulva and B. nivea strains. The effect of heat treatment intensity on outgrowth of B. fulva strains was more apparent at the most intense heat treatment evaluated (90 degrees C/10 min), which was also the condition in which greater dispersion of the estimated kinetic parameters was observed. On the other hand, B. nivea strains were more affected by heating, resulting in greater variability of growth parameters estimated at different heating intensities and in very long lag phases (up to 25 days). The results show that inter- and intraspecies variability in the kinetic parameters of Byssochlamys sp. needs to be taken into account for more accurate spoilage prediction. Furthermore, the effect of thermal treatments on subsequent outgrowth from ascospores should be explored in combination with other relevant factors such as degrees Brix and oxygen to develop thermal processes and storage conditions which can prevent the growth of HRMs and spoilage of heat treated food products.