Summary:The effect of acidity on the growth ofCl. botulinumwas studied by following changes in the viable count of cultures in acid buffered broths containing bread crumbs, after inoculation with about 104vegetative cells or spores/ml. In 9 out of 10 experiments growth did not occur below pH 5.0; in the tenth it took place at pH 4.8. There was no significant difference between vegetative cells and spores.