Examination of samples received in 1947 and 1948 showed that a subepidermal mycelium occurred in normal wheat grains from almost all the wheat-growing areas of the world. The amount of mycelium varied widely: there are indications that the degree of infection is dependent on the atmospheric humidity during the ripening of the grain. No subepidermal mycelium was found in wheat grains from some crops grown under irrigation. (P < 0'001).* Investigations in 1948 on Bers6e wheat at Slough and on a number of varieties at Heston had shown that it is at this comparatively late stage, namely, after the ears have turned yellow and the grains begun to dry out, that the subepidermal fungi are first seen.