Two new alleles, C and c, involved in mating type expression were demonstrated. A dominant allele, C(cycler), must be present for the expression of the rhythm involving a sequential alternation of the two complementary mating types (III and IV). Cultures can be entrained with light-dark cycles. The phase of each clone can be characterized by its III to IV and IV to III transitions in relation to the zero hour of a given light-dark cycle. Phase is a stable phenotypic trait during asexual reproduction, but following sexual reproduction it does not display Mendelian segregation. Instead phase is determined through nuclear differentiation, i.e., the trait is controlled by differently determined macronuclear alagen (caryonidal inheritance) which normally segregate at the second cell division after conjugation. The phase of a clone within its genetic limits is a function of the photofractions and the light intensities used in the entraining treatment. By examining a number of clones a variety of phase angles between the mating type cycle and the entraining light-dark cycle are found. Dividing cells which are sexually unreactive and therefore do not express the rhythm can be entrained and following entrainment, phase is inherited through repeated cell replications at a rate greater than one fission a day in continuous darkness or continuous dim light. This result unique to this system indicates that the cellular processes underlying the phase and period of this circadian rhythm persist (unexpressed: sexual reactivity requires slight starvation) through repeated cell replications even when the division cycle is considerably shorter than the expressed circadian period. The rhythm has a circadian period in continuous darkness or light (tested for six days) of less than 24 hours. The reversal of mating type ceases in continuous light at higher intensities. Cells homozygous for the recessive allele, c(acyclic), do not reverse mating type but are either mating type III or IV, again as a consequence of nuclear differentiation. Since individual cells with the dominant allele express both mating types, differentiation for mating type can not involve the absence in the macronucleus of mating type determining factors.