Previous studies have defined 29 genes necessary for synthesis of the Escherichia coli flagellar apparatus. This study analyzed the transcriptional control of flagellar genes, using Mu d (Apr lac) phage to generate flagellar mutants by insertion. These mutants contained operon fusions of flagellar genes to the lac genes of the Mu d phage and allowed the measurement of flagellar operon expression by detection of beta-galactosidase activity. These fusion mutants expressed the enzyme activity constitutively, and an autogenous regulation mechanism was not revealed. Lambda transducing phages carrying these chromosomal fla-lac fusions were also isolated and used to examine the effect of different fla mutations on expression of each flagellar operon. The results showed that flagellar operons are divided into six classes; (class 1) the flbB operon, which controls all of the other flagellar operons; (class 2) the flaU and flbC operons, which are controlled by the flbB operon gene products and are not required for the expression of other Fla operons; (class 3) the flbA, flaG, flaD, flaN, flaB, and flaA operons, which are under flbB operon control and are required for the expression of other fla operons; (class4) the flaZ operon, which is controlled by the gene products of the group 1 and 3 operons and is required for hag transcription; (class 5) the mocha and flaS operons, which are controlled by the gene products of the group 1 and 3 operons; and (class 6) the hag operon. These results are discussed with respect to the possible assembly sequence of the fla gene products.