SUMMARY.1. Certain seedlings of the wild hop when grown in the greenhouse are persistently immune to the attacks of the mildewSphaerotheca Humuli.This immunity has been shown by the same individual seedling for three consecutive years. Under the same cultural conditions other seedlings of the same parentage prove to be very susceptible.2. Certain seedlings (Group 2) which are immune when grown in the greenhouse are also immune when grown in the open. These seedlings have retained this immunity after four years' residence in a hop‐garden under normal conditions of cultivation and manuring.3. Certain seedlings (Group 3) which are immune when grown in the greenhouse are susceptible when grown in the hop‐garden; in some cases the susceptibility shown is of the highest grade.4. “Cuts” taken from the seedlings of Group 3 in the same year in which the seedling proved to be susceptible in the hop‐garden are immune in the greenhouse under cultural conditions in which “cuts” taken from other susceptible seedlings in the hop‐garden are very susceptible.5. Certain seedlings (Group 4) are semi‐immune to the attacks of the mildew.6. One seedling (of American ancestry) grown in the greenhouse was immune throughout the season in 1916 and very su