Exploring the evolutionary response of Bacillus simplex strains to the slope-specific habitats of 'Evolution Canyon' I and II, Israel, we report here on presumably adaptive differences in fatty acid (FA) content that correlate with one particular feature of the habitats, temperature difference. These two canyons represent similar ecological sites, separated by 40 km, in which the orientation of the sun yields a strong sun-exposed and hot 'African' south-facing slope versus a rather cooler and mesic-lush 'European' north-facing slope within a distance of only 50-400 m. Among 131 strains, which are identical in their 16S sequences, those assigned genetically to the 'African' ecotypes express phenotypically generally more high-temperature-tolerance-providing iso-branched FAs than strains assigned to the 'European' ecotypes when grown at 20 degrees C, 28 degrees C and 40 degrees C. Conversely, 'European' lineages express larger amounts of low-temperature-tolerance-providing anteiso-branched and non-saturated FAs when grown at the same temperatures. Moreover, 'African' ecotypes show a stronger adjustment of their high- and low-temperature-tolerance-providing FAs in response to low temperatures, which suggests that, as a result of temperature adaptation, 'African' and 'European' ecotypes have evolved different reaction norms within their phenotypic plasticity response. Thus, bacterial adaptive microevolution may include such multigenic and highly complex organs as the bacterial cell membrane. The results contribute to our understanding of the speciation process among the 'Evolution Canyon' B. simplex ecotypes.