The infrared absorption spectra of the aerobic spore-forming bacilli are variable. Evidence is presented to show that, for Bacillus megaterium, the nature of the spectrum is related to the morphology of the bacteria, both showing a cyclic variation with time. The time scale of the cycle is affected by the temperature at which the cultures are incubated, by their population density and by the medium on which they are grown; it may be affected by induced drug resistance. Some of the characteristics of the spectrum of vacuolated vegetative organisms are due to the presence of an extractable substance which may be a polymer of β-hydroxybutyric acid. Difference spectroscopy is used to demonstrate the presence of this substance, its disappearance as the bacteria autolyse and the appearance of β-hydroxybutyric acid in the autolysed material. This technique is applied also to establish the state of dipicolinic acid in the resting spores of B. megaterium. Any application of infrared spectroscopy to the study of aerobic spore-forming bacilli must take account of the morphological state of the bacteria.