SUMMARY Incorporation of radioactive glucose and N-acetyl glucosamine into the walls of growing fungal hyphae has been studied using light microscopic autoradiography. Autoradiographs of Phytophthora parasitica with glucose as substrate and Neuro- spora crassa and Schizophyllum commune with glucose or N-acetyl glucosamine as substrate show that most of the incorporation after incubation times as short as I min. is at the extreme hyphal tip. N-Acetyl glucosamine is incorporated less into the subapical region than glucose, and N. crassa shows less subapical incorporation of both substrates than the other two fungi.