The region for intrinsic transcription termination of the Escherichia coli rnpB gene coding for M1 RNA is repeated three times. Each region encodes an RNA-terminator hairpin and U-rich 3' tail. Most transcription is terminated at the first terminator (T1), but a complete termination requires the second terminator (T2) and the third (T3). A deletion experiment, where deletions were extended from T3 into T1, was carried out to determine parameters affecting in vivo transcription termination at T1. The deletion of T3, T2 or the downstream sequences up to near the termination site of T1 showed little or no significant effect on transcription termination at T1. When the base corresponding to the acute termination position of T1 was included in the deletion, termination efficiency slightly decreased. For T1 bearing the deletion of the region encoding the RNA-terminator hairpin and U-rich 3' tail, termination was completely abolished. When T1 contained only the region encoding the RNA-termination hairpin without the U-rich 3' tail, termination still occurred even though the efficiency was low. Characteristics of rnpB termination were discussed with these results.