Neurospora crassa is an organism well suited for the study of mitochondrial biogenesis. It has a relatively prolonged logarithmic growth phase, with mass doubling times of about two hours. During the exponential growth period the mitochondrial mass doubles at the same rate as does the culture. Highly purified mitochondrial fractions can be isolated from the cells using equilibrium centrifugation in sucrose gradients (Luck, 1963a). A mutant strain requiring choline incorporates radioactive choline into the lecithin of mitochondria, and when the choline of the growth medium is changed to the non-radioactive compound, the radioactive choline of the mitochondria is retained there and can serve as a stable marker. We made use of these features of choline requiring Neurospora to find out how pre-existing membrane material is distributed among individual mitochondria in a growing population (Luck, 1963a,b). Cultures labeled with tritiated choline were transferred to unlabeled medium and permitted to grow for three doubling cycles. The distribution of label among individual organelles was measured by quantitative radioautography. The results showed that throughout three doubling cycles of the mitochondrial mass the label was uniformly distributed among all mitochondria. These studies gave support to the hypothesis that in exponentially growing cells the mitochondrial mass grows by addition of new material to an existing framework and the mitochondrial population increases by division. Recent studies with the choline requiring mutant, a leucine requiring mutant, and a wild-type strain, indicate that exponentially growing cells may vary the chemical composition of their mitochondria within substantial limits (Luck, 1965a). The variation affects the phospholipid to protein ratio, and may be brought about by change in the available level of amino acids or choline present in the growth medium. In a cholineless strain, for example, two levels of choline supplementation can be defined: one which is just adequate to support exponential growth at maximal rates, and another, ten times higher, which gives maximal incorporation of choline into cellular phospholipids but does not alter the growth rate. Mitochondria produced by the culture at these two choline levels differ in their density. Figure 1 is a photograph of centrifuge tubes containing continuous sucrose gradients. Mitochondria have been sedimented to their equilibrium position. Cultures growing at low choline conditions produce mitochondria (tube A)