The genes encoding the structural components of the Caulobacter crescentus flagellum are temporally controlled and their order of expression reflects the sequence of assembly. Transcription of the operon containing the structural gene for the flagellar hook protein occurs at a defined time in the cell cycle, and information necessary for transcription is contained within a region between -81 and -120 base-pairs from the transcription start site. To identify the sequence elements that contribute to the temporal control of hook operon transcription, we constructed deletions and base changes in the 5' region and fused the mutagenized regulatory region to transcription reporter genes. We demonstrate that sequences 3' to the transcription start site do not contribute to temporal control. We confirm that upstream sequences between -81 and -120 base-pairs are necessary for temporal activation, and that transcription also requires sequences at -26 to -46 base-pairs. A specific binding activity for the region between -81 and -122 base-pairs was shown to be temporally controlled, appearing prior to the activation of hook operon transcription. This binding activity was missing from strains containing mutations in flaO and flaW, two genes near the top of the flagellar hierarchy known to be required for hook operon transcription. Thus, the hook operon upstream region contains a sequence element that responds to a temporally controlled trans-acting factor(s), and in concert with a second sequence element causes the timed activation of transcription.