Slit-scanning flow cytometry allows analysis of the locations Of components along the long axes of cells. This is useful in determining how the components of a dividing cell are partitioned into the two daughter cells. Ciliated protozoa, unlike most eukaryotic cells, do not partition their:DNA equally between daughter cells. We have used slit-scanning flow cytometry to measure the distribution of this unequal DNA partitioning in the ciliate Tetrahymena pyriformis. Our results, show that the difference in the amounts of DNA alloted to sister nuclei varies from cell to cell but, on the average, increases with the DNA content of the mother cell. However,the average difference in the fraction of the mother cell DNA partitioned to each daughter cell is more or less independent of the DNA content of the mother cell and is about 8.5% of that DNA content. Slit-scanning flow cytometry also allows determination of the DNA distributions of dividing and newborn cells, determinations that are difficult to make with good statistical precision by other means. The measured newborn and dividing cell DNA distributions are broad, and this must be the result;of repeated rounds. of unequal partitioning of DNA.