Pilus assembly in CAULOBACTER: crescentus occurs during a short period of the cell cycle and pili are only present at the flagellar pole of the swarmer cell. Here we report a novel assay to visualize pili by light microscopy that led to the purification of CAULOBACTER: pili and the isolation of a cluster of seven genes, including the major pilin subunit gene pilA. This gene cluster encodes a novel group of pilus assembly proteins. We have shown that the pilA promoter is activated late in the cell cycle and that transcription of the pilin subunit plays an important role in the timing of pilus assembly. pilA transcription is regulated by the global two-component response regulator CtrA, which is essential for the expression of multiple cell cycle events, providing a direct link between assembly of the pilus organelle and bacterial cell cycle control.