The aging characteristics of various known mutations of Paramecium tetraurelia have been studied. Among the mutations which had been previously isolated on the basis of the inability of the cell to do exocytosis of its trichocysts, one group was identified which had clonal life spans which were indistinguishable from, or were somewhat shorter than, the life span of wild-type. Another group of trichocyst mutations, however, had clonal life spans which were 1/3 to 1/4 that of wild-type. Further study of the phenotypes of these various mutations revealed that members of the short-lived group all had a common phenotypic defect: a frequent inability to properly divide the macronucleus during cell division. Another mutation which has normal trichocysts, but which shows macronuclear division defects, also has an extremely short life span. Thus, the short clonal life spans seen are not associated with the inability to do trichocyst exocytosis but rather are associated with the inability to properly divide the macronucleus. An hypothesis is presented which relates macronuclear misdivision to the short clonal life spans of the mutants expressing it. By extension of the hypothesis, the significance of normal macronuclear divisions to the normal aging process in P. tetraurelia is proposed.