By making use of Zn65 the amt. of Zn necessary to obtain max. growth of Neurospora crassa was studied. Max. growth requires that at least 1/30,000 part of the dry weight of the Neurospora be composed of Zn, and can be obtained by raising the mold in a culture soln. containing 0.08 mg. or more of Zn/l. When placed in a culture soln. containing inactive Zn more labelled Zn migrates from Neurospora into the soln. at 12[degree]C than in the absence of Zn. Cobalt uptake, studied by making use of labelled cobalt (Co58), amounts only to 5% of the uptake of Zn under corresponding conditions.