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When various wild strains of Penicillium lanosum and Aspergillus niger were placed in the same mild laboratory environment, their frequencies of new spontaneous mutations were clearly related to whether they had been isolated from a region of high or low microclimatic stress. In the mild environment, the total frequencies of conidial color and morphological mutations in P. lanosum , summed over all relevant loci, ranged from 0.29% to 2.4% for six strains from the north-facing, less stressful “European” slope (ES/NFS) of “Evolution Canyon” I, compared with 6.5–11.6% for five strains from the south-facing “African” slope (AS/SFS), which is a much more stressful environment, being harsher, drier, more fluctuating in temperature, and receiving up to eight times more UV radiation than the opposite slope. The corresponding figures for A. niger were 0.42–1.50% for three strains from the ES/NFS and 2.3–4.9% for six strains from the AS/SFS. The more mutagenic environment of the AS/SFS than of the ES/NFS means that, in Evolution Canyon, the mutation frequency differences between the very stressful environment and the less stressful environment are probably even larger than the 4- and 6-fold differences found here in a mild laboratory environment. The evidence from these two filamentous fungi, which have no sexual cycle, is that there are inherited differences in spontaneous mutation rates according to the levels of stress in the environment, and this feature may well be adaptive. Evolution Canyon I is at Nahal Oren, Mount Carmel, Israel.