A slow in vivo uptake of cobalt from a growth medium resulted in an increase in density of mitochondria ofSchizophyllum commune. Differential labeling of donor and resident mycelia, and subsequent analysis of resident mycelia surrounding donor implants, detected cobalt- dense mitochondria and demonstrated exchange of mitochondria after hyphal fusion. Transfer of mitochondria occurred in fully compatible, common-A, and common-AB matings, but was not detected in common-B matings of the tetrapolar Basidiomycete S commune.