In the course of experiments on the cultivation of anaerobic bacilli, it was found advisable to be able to calculate with some certainty on the production of spores by the strains of anaerobes worked with. Observations were accordingly made of the conditions of cultivation by which sporulation appeared to be influenced. It was evident that in the case of B. sporogenes and other well-known types observed, the proportion of spores produced within a given period varied markedly in response to variations in the same factors that mainly influenced the rate of growth in cultures, viz. concentration of food material, incubation temperature, and reaction of medium. With the exception of cultures in media containing added carbohydrate, it was evidently also the rule that sporulation was most active in cultures commencing under conditions most favourable to rapidity of vegetation.