Aims: This study was to illustrate the relationship between the thermotolerance and the contents of hydrophobin-like or formic-acid-extractable (FAE) proteins in aerial conidia of Beauveria bassiana and Paecilomyces fumosoroseus produced on rice-based substrate. Methods and Results: Survival indices of 11 isolates were separately assessed as a ratio of the viability of conidia after 3-150 min thermal stress at 48degreesC over that of unstressed conidia and fitted well to a survival model (r(2) greater than or equal to 0.97). For a given isolate, the fitted model generated an LT50, the time for 50% viability loss under the stress. The LT(50)s of six B. bassiana isolates (10.1-61.9 min) and five P. fumosoroseus isolates (2.8-6.2 min) were correlated (r(2) = 0.81) with FAE protein contents (6.9-23.4 mug mg(-1)). The survival indices of a fixed B. bassiana isolate after 45-min thermal stress at 48degreesC were also correlated to the FAE protein contents from conidia produced on glucose-, sucrose-, or starch-based substrate (0.79 less than or equal to r(2) less than or equal to 0.86). Three bands of 12.0, 15.0 and 17.5 kDa were recognized from SDS-PAGE profiles of protein extracts from different isolates or from the fixed isolate grown on the three carbon sources, but the 12 kDa was less likely related to the thermotolerance. Conclusions: Approximately 80% of variability in conidial thermotolerance was attributed to either 15.0- or 17.5-kDa FAE protein or both. Significance and Impact of the Study: The relationship between conidial thermotolerance and FAE protein contents implicates a new way to search for fungal biocontrol agents with more thermotolerance and environmental persistence.