We report here that, using Northern (RNA) blots, we identified two relatively stable transcripts of 4.6 and 1.1 kb that correspond to the products of the ilvEDA and ilvE genes and two relatively unstable transcripts of 6.7 and 3.6 kb that correspond to the products of the ilvGMEDA and ilvDA genes. The transcripts were identified by the use of eight probes derived from segments of the ilvGMEDA cluster. In addition, we used two strains with deletions of ilvG or ilvDA and observed the expected decrease in transcript size in Northern blots. Primer extension with reverse transcriptase generated a 169-nucleotide product corresponding to a 5' end within the ilvED intercistronic region, 37 nucleotides from the AUG codon of the ilvD gene. This primer extension product presumably indicates the 5' end of the ilvDA transcript that we detected in Northern blots. The stability of the transcripts was monitored, and RNase E was found to play a major role in ilv transcript degradation. Transcript levels varied in response to growth in the presence of the end product amino acids and in response to the presence of the polar frameshift site in ilvG. Although there have been speculations about the identities and numbers of transcripts derived from the ilvGMEDA cluster on the basis of the identification of some of the sites of transcription initiation and termination, this is the first report of the use of Northern blots to determine the actual sizes and distribution of mRNAs present in vivo.