In previous work, at least 10 discrete cistrons involved in the synthesis of flagella in Escherichia coli were described. Six cistrons were located between his and uvrC on the genetic map. These were referred to as hag, flaA, flaB, flaC, flaD, and flaE. Four cistrons referred to as mot, flaG, flaH, and flaI were located between uvrC and aroD. In order to determine whether these genes are organized into transcriptional units, a series of Mu phage-induced flagellar mutants was studied. The mutant strains behaved as if they were carrying strong polar mutations. Of 228 independent Mu-induced mutants, 114 with mutations in the his-aroD region of the genetic map were tested by preparing partial diploid strains with episomes carrying a variety of previously defined mutations. The pattern of complementation that emerged indicated that cistrons flaB, flaC, and flaE form a transcriptional unit. Cistron flaO, defined in the course of this study, is also a member of this transcriptional unit. The order of transcription is B-C-O-E. flaA was found to be complex, and it included four cistrons, flaA, flaP, flaQ, and flaR, with the transcriptional order A-P-Q-R. Cistrons flaG and flaH are cotranscribed with the transcriptional order G-H. The remaining genes, flaD, flaI, hag, and mot do not belong to multicistronic transcriptional units. Complementation analysis suggested that the cheC locus is the same as cistron flaA.