Chlorella sp. UK 001 cells were cultivated under a photoautotrophic condition, utilizing carbon dioxide and light as carbon and energy sources, respectively. In batch cultures of the cells for 96 h at incident light intensity (I-0) of 21.8 W m(-2), when the culture temperature (T) was kept at 40 degrees C, mean diameter of the cells (<(d)(c) over bar>) increased from 2.3 mu m (standard deviation (SD) = 0.36 mu m) to 11 mu m (SD = 1.9 mu m), while at T = 20, 30, 35 and 45 degrees C, the Values of a hardly changed. Under the conditions examined, the appreciable enlargement of the cells occurred at T = 40 degrees C and at I-0 = 16 to 46 W m(-2), and was not accompanied by a substantial increase in the number of cells. From the data on cell viability, oxygen generation rate under light conditions and oxygen consumption rate under dark conditions, the enlarged cells in the culture at 40 degrees C were found to have sufficient activities as living cells. Sedimentation experiments were carried out by standing the cells which were cultivated at 30 and 40 degrees C. The interfacial sedimentation rate of the cells ((d) over bar(c) = 11 mu m, SD = 1.9 mu m) obtained from the culture at 40 degrees C was about twelve times higher than that of cells ((d) over bar(c) = 2.3 mu m, SD = 0.38 mu m) at 30 degrees C.