Batch and repeated-batch fermentation tests with a 5-l jar fermenter were carried out to investigate the relationship between cell viability and the intracellular trehalose content of a thermotolerant strain of flocculating yeast, Saccharomyces cerevisiae KF-7. During batch fermentation at 35 degrees C, no trehalose was detected for the first 8 h; however, trehalose content increased thereafter, reaching a maximum at the stationary phase. The trehalose contents of viable cells at the stationary phase during batch fermentation at 30, 33, and 35 degrees C were 0.5, 0.8, and 1.2 mu g/10(8) cells, respectively, which were almost the same as those of repeated-batch fermentation at the same each fermentation temperatures. Thus, the trehalose content of viable cells at the stationary phase increased with increasing fermentation temperature. However, after completion of the fourth round of repeated-batch fermentation, the trehalose content and the number of viable cells decreased with time at all fermentation temperatures. At 30, 33, and 35 degrees C, the decay constants, kd, were 0.021, 0.038, and 0.13 h(-1), and rates of decrease in trehalose content, k(Tr), were 0.016, 0.036, and 0.110 mu g/10(8) cells . h, respectively. It was apparent that both k(d) and k(Tr) increased with increasing fermentation temperature.