Abstract Growth rates of Tetrahymena pyriformis GL are reported following temperature shifts and during induction of synchronous cell division. Use of an electronic cell counter permitted the ready determination of cell volume distributions. These distributions were used in determining growth rates. Tetrahymena pyriformis GL was cultured in proteose peptone yeast extract medium at 28.0, 30.8, 32.5, and 33.9 °C. Following a temperature shift from 28 °C to a higher temperature, a brief period of continued cell division occurred followed by a prolonged period of blocked cell division. Division block was permanent at 33.9 °C and was of a temporary nature during the temperature adaptation phase at 30.8 and 32.5 °C. Correlation coefficients of the growth rates suggest that cell growth during temperature adaptation was exponential. Cell volumes were log-normally distributed before the temperature shift and during periods of temperature adaptation. The slope of the log volume class distribution was constant with time during the period of inhibited cell division. During induction of synchronous division, produced by intermittent heat shocks, cell growth was also exponential, cell volumes were log-normally distributed and the slope of the log volume class distribution was constant. This constancy in slope during periods of inhibited cell division indicates that the rate of cell growth is independent of the division cycle during single and multiple temperature adaptation. During periods of inhibited cell division the rate of cell growth at 30.8 °C was significantly different from that at 32.5 °C. However, the rate of growth at 33.9 °C was significantly lower than at 30.8 or 32.5 °C. This indicates that the optimum temperature for the rate of cell replication and growth are different.