Mutations in CDC genes of S. cerevisiae disrupt the cell cycle at specific stages. The experiments reported here demonstrate that two CDC genes, CDC5 and CDC27, are necessary for mitochondrial segregation as well as for nuclear division. The defect in the transmission of mitochondria was revealed by the examination of uninucleate and binucleate progeny of transient heterokaryons generated by using the kar1-1 mutation that disrupts nuclear fusion. One of the parents lacked mitochondrial DNA (ρ0) whereas the other parent had functional mitochondria (ρ+). When the parents of the heterokaryon were both wild-type (CDC), nearly all progeny received mitochondria at 21° and at 34°. Thirty-four of the 36 cdc mutations tested had no defect in transmission of mitochondria to zygotic progeny in crosses in which one parent was a cdc mutant and the other parent was not (CDC). However, the cdc5 and cdc27 mutations prevented the transmission of mitochondria to cdc progeny at 34° but not at 21°; CDC progeny received mitochondria at either temperature. This defect was observed in crosses of cdc5 or cdc27 by wild-type cells regardless of which parent donated mitochondria to the zygote. The defect in mitochondrial transmission cosegregated in meiotic tetrads with the defect in mitosis demonstrating that both are likely to be caused by the same temperature-sensitive mutation. These results indicate that the CDC5 and CDC27 gene products are essential in two motility-related processes: mitochondrial movement from the zygote to the progeny and in mitosis.—Furthermore, the results suggest that the function performed by the CDC5 and CDC27 gene products for mitochondrial transmission differ in some fundamental way from the function performed for mitosis. The function necessary for mitosis can be supplied to the cdc5 (or cdc27) nucleus by the CDC5 (or CDC27) nucleus in the same heterokaryon but the function necessary for mitochondrial transmission cannot. Perhaps the function needed for mitochondrial transmission must be performed in the cell cycle preceding the actual segregation of mitochondria whereas the function needed for nuclear segregation can be performed at the time that mitosis occurs.