A theoretical relationship is developed relating growth of a fungus during one cycle of a diurnal temperature fluctuation to the growth at relevant constant temperatures. The theory involves the assumption that the growth‐rate of the fungus at a given temperature in the cycle is unaffected by its previous temperature experience. Growth of Gibberelta zeae, Cochliobolus sativus and C. spicifer was measured at each of sixteen diurnal temperature regimes differing in median temperature and amplitude. The measured growth was compared with the expected growth. In general, the analysis supports the derived theoretical relationship. In some instances, growth at a fluctuating temperature is greater than or less than that at the corresponding median and the difference is anticipated, and explained, by the theory. Copyright © 1968, Wiley Blackwell. All rights reserved
