Asymmetric cell division in Caulobacter crescentus produces two cell types, a stalked cell and a new swarmer cell, with characteristics surface structures. We have examined the role of the cell cycle in the differentiation of these two cells using adsorption of bacteriophage phi LC72, the assembly of the polar flagellum, and stalk formation as assays for changes in surface morphology. Previous studies of this aquatic bacterium [17,25] have suggested that the replicating chromosome acts as a "clock' in timing the formation of the flagellar filament at one pole of the new swarmer cell. the analysis of conditional cell cycle mutants presented here extends these results by showing that DNA synthesis is also required for adsorption of phage phi LC72 and, more importantly, they also suggest that a late cell division step is involved in determining the spatial pattern in which the phage receptors and flagella are assembled. We propose that this cell division step is required for formation of "organizational' centers which direct the assembly of surface structures at the new cell poles, and for the polarity reversal in assembly that accompanies swarmer cell to stalked cell development.