Summary: Physiological studies recently led to the development of a technique for obtaining synchronous nuclear division in Salmonella typhimurium. By using this method we have attempted to identify the morphological changes characteristic of nuclear division. The picture obtained is that of a gradual shift from a round nuclear body via ‘wedge’- and ‘bar’- to ‘dumbbell’-like configurations. Mitotic patterns were not observed in these small cells. An indication was obtained that at an early stage of nuclear division the nucleus has already doubled with respect to at least one of its physiological functions. It is discussed to what extent the present studies by selective staining may be used to identify the state of the nucleus as observed by electron microscopy.