The mean time-to-death (t) of imaginal Drosophila of an inbred line in alcohol vapor of constant partial pressure (P) is a declining rectilinear function of P for each age. The time-to-death depends upon the diffusion into the fly of an amount of alcohol sufficient to kill. It does not depend upon any measurable property of a reaction between the substance of the fly and the alcohol which produces death. The relation between t and P is independent of temperature, but the invasion coefficient S = Deltat/DeltaP declines with age and differs for the two sexes. The first derivative of S with respect to age exhibits sharp discontinuities. The internal alcohol required to kill declines with age, varying with S. The relative variation of t, sigma(t)/t, is directly proportional to the resistance to diffusive penetration of alcohol R, where R = 1/S. The vapor pressure of alcohol estimated to kill instantaneously shows periodic fluctuations with age; these are precisely correlated with changes in the slope of S as a function of age. Periodic fluctuations of invasibility by alcohol, and of the lethal dose, are interpreted as due to the incidence of suppressed moults. It is shown that in the accumulation of deaths as a function of time (age) in a genetically uniform population of Drosophila of one sex, similar fluctuations are apparent in the rate. The statistical smoothing of such data is not legitimate.