Rhodopseudomonas palustris was chosen as a model organism for studying bacterial differentiation. Synchronous populations selected by sucrose gradient centrifugation yielded more than 95% swarmer cells. The appearance and disappearance of cell morphological groupings and the doubling of cell numbers in cultures of such swarmer populations were very well defined. Cells were only motile for the first half of the division cycle, but motility was regained before division. Development gave rise to a distinct and characteristic pattern of extinction increase and particle volume distribution. The development of swarmers into mother cells and the dimorphic division of R. palustris are discussed as simple examples of differentiation.