Endogenous circadian rhythms of spontaneous bioluminescence are demonstrated in two recently collected dinoflagellate strains,Pyrocystis elegansstrain ME 8 andPyrocystiscf.acutastrain ME 9. The rhythms freerun in DD for several periods, but are readily damped out in L.L. The emitted light intensity allows measurements in single cells by means of a scintillation spectrometer. With regard to the periodicity of spontaneous bioluminescence, the two species differ from other cultured strains ofPyrocystisspecies such asP. lunulaandP.fusiformis, which only show rhythms of induced bioluminescence.