A short-period autonomous respiratory ultradian oscillation (period Ϸ 40 min) occurs during aerobic Saccharomyces cerevisiae continuous culture and is most conveniently studied by monitoring dissolved O 2 concentrations. The resulting data are high quality and reveal fundamental information regarding cellular dynamics. The phase diagram and discrete fast Fourier transformation of the dissolved O 2 values revealed a square waveform with at least eight harmonic peaks. Stepwise changes in temperature revealed that the oscillation was temperature compensated at temperatures ranging from 27 to 34°C when either glucose (temperature quotient [Q 10 ] ؍ 1.02) or ethanol (Q 10 ؍ 0.82) was used as a carbon source. After alteration of the temperature beyond the temperature compensation region, phase coherence events for individual cells were quickly lost. As the cell doubling rate decreased from 15.5 to 9.2 h (a factor of 1.68), the periodicity decreased by a factor of 1.26. This indicated that there was a degree of nutrient compensation. Outside the range of dilution rates at which stable oscillation occurred, the mode of oscillation changed. The oscillation in respiratory output is therefore under clock control.Oscillatory dynamics, which range from those observed in particle physics to those of yearly clocks, are ubiquitous, and in most cases knowledge concerning the underlying processes that dictate the time base, synchronization, and regulation of systems remains rudimentary. In living organisms dynamic behavior provides a unique window through which the intricate spatiotemporal organization of cells can be viewed.Clocks are universal and fundamental to living organisms and are the basis of temporal control of metabolism and behavior (20). The majority of research has focused on the daily clock (circadian clock) that is found in the entire range of organisms from cyanobacteria to humans (6, 9). However, several classes of shorter-period temperature-compensated clocks also exist. Examples of these clocks include the millisecond clock observed during the courtship of Drosophila (19), the 40-s defecation clock (fast clock) found in nematodes (10), and the ultradian clock (period Ϸ 1 h) found in Acanthamoeba castellanii (21) and in Paramecium tetraurelia (15). Biological clocks can be differentiated from biological oscillators and rhythms by two properties: they must run continuously under constant conditions and have temperature compensation (6,20,24) The effects of temperature on the frequencies of biological oscillations have been extensively studied (25, 32). The periods of glycolytic oscillators (4) and cell cycle oscillators (1) are temperature dependent, and the oscillation periods are usually halved when there is a 10°C increase in temperature; i.e., the temperature quotient (Q 10 ) is ϳ2. Such oscillators have no intrinsic timekeeping function, although clocks can drive them (16). Temperature-compensated oscillators have a Q 10 of ϳ1;i.e., there is little alteration in the period when the temperature changes (...