Increased concentrations of yeast extract led to increased growth yields and faster growth rates of the newly isolated Clostridium thermobutyricum. This species produced butyrate as its main fermentation product from glucose as well as from yeast extract. In the presence of peptone or tyrptone and during growth on agar, up to 70% of the cells sporulated. Growth yields were 30 and 55 g per mole glucose in the presence of 0.05 and 2.0% yeast extract, respectively. The Arrhenius graph was biphasic, exhibiting an intermediary plateau around $38^{circ}C$ with a concomitant change in the Arrhenius energy. The optimum temperature was $55^{circ}C$. An unusually sharp decline in the growth rate occurred above $59^{circ}C$ .