We have localized the antitermination system involved in E. coli ribosomal RNA transcription and compared it with antitermination in the lamboid bacteriophages. In vivo experiments with gene-fusion plasmids were used to examine the ability of specific areas of the rrnG control region to convert an ordinary transcription complex into antitermination transcription complex. A 67 bp restriction fragment immediately following the rrnG P2 promoter decreased transcription termination about 50%. This fragment contains box A-, box B-, and box C-like sequences similar to those in lambda nut loci. It also caused transcripts from lac and hybrid trp-lac promoters to read through a transcription terminator. Translation through the 67 bp segment or reversal of its orientation resulted in complete loss of antitermination activity. We conclude that the E. coli ribosomal RNA operons possess an antitermination system similar to that used by the bacteriophage lambda.