Experiments consisted of three phases: spore production, spore storage and spore germination. Throughout the experiments temperature was near-optimal. In each phase various light and humidity treatments were given. The time at which a spore germinated was considered to be a stochastic variable. 3,000 lux inhibits germ tube elongation measurably; 7,600 lux inhibits germination measurably. Spores produced in darkness are more sensitive to light than spores produced in light, independent of the age of the pustules. Hydration of spores during storage increases light-sensitivity during germination. Samples of spores formed in darkness at low humidity are considered to consist of three sub-populations: a light-insensitive one presumably having profited from the foregoing light period, a light-sensitive sub-population and a group of spores which is inert or dead. The light-inhibited spores germinate rapidly after being placed in darkness with a germination rate little affected by the duration of light exposure. After four hours of light-exposure a gradually increasing proportion of the light-inhibited spores is killed, which means that the reversibility of the light-inhibition is limited.