Adaptation to novel environments usually entails morphological changes. The cell morphology of six experimental populations of Pseudomonas pseudoalcaligenes and their common ancestor were examined with scanning electron microscopy (SEM). The six experimental populations were propagated under different temperatures for 10 months: three of them cultured at constant normal temperature (35degreesC) forming the control group, and the other three cultured at incremental higher temperatures (from 41degrees to 47degreesC) as the HT group. SEM showed the deformed and elongated cells in the 6-h cultures of both ancestral and control populations at 45degreesC, indicating that 45degreesC is stressful for the ancestral and the control populations. In contrast, the HT populations retained normal cell shape in the 6-h cultures at both 35degreesC and 45degreesC. The mean cell volumes of control and HT populations increased 29% and 34%, respectively, relative to the ancestor at their respective thermal regimens, suggestion that the culturing conditions might favor larger cells.