Experiments were performed to identify relationships between morphological and physiological events during batch fermentation of Clostridium acetobutylicum ATCC 824. Differing fermentation conditions were obtained by manipulation of the culture pH value during the process. The bacterium showed marked changes in morphology during its cultivation, similar to those previously observed for other strains. However, although the acidogenic phase was characterized by the presence of rod-shaped cells, and the solventogenic phase by clostridial forms, there was no simple relationship between the proportion of clostridial forms present and the ratio of solvents to acids. Nevertheless, the shift from acidogenesis to solventogenesis was always coincident with the presence of some clostridial forms and with the accumulation of granulose within the cells. In addition, the "solvent shift" was associated with major changes in the cellular protein pattern, as analysed by sodium dodecyl sulphate/polyacrylamide gel electrophoresis. Hence, the potential solventogenic ability of any particular culture may be recognised by its morphological appearance and/or its cellular protein pattern.