Populations of Paramecium bursaria cells display a circadian rhythm of photoaccumulation. Both Chlorella-containing cells and Chlorella-free cells exhibit this rhythm. Several other species of Paramecium do not express rhythmic photoaccumulation when tested under the same conditions. In P. bursaria, photoaccumulation rhythms persist in continuous conditions (constant temperature and either continuous light or continuous darkness). The period of this rhythm is "temperature-compensated," with a Q10 of 1.10-1.12. The rhythm can be reset by pulses of light or darkness in a phase-dependent manner. Therefore, an endogenous circadian oscillator controls photoaccumulation behavior in P. bursaria.