RNA polymerase I transcripts, purified from Schizosaccharomyces pombe cells, terminate at three sites that precede 'Sal box'-like termination element (TE) sequences. Essential features in these elements were investigated by the in vivo expression of targeted mutations. RNA analyses confirmed a functional significance for two of the elements (Boxes 1 and 3), but indicated that the third, less related, sequence (Box 2) does not function as a termination signal. The results further indicated that the most conserved residues in the two active TEs, as well as adjacent regions, are also most critical to function. Furthermore, some mutations in these elements or in immediately flanking sequences affect not only the efficiency of termination, but also alter the position of termination by as much as 35 nt. Since the element is able to influence the site of termination over a surprisingly long stretch of DNA sequence, these observations suggest that the TE does not act simply as a pause element by fixing the termination factor.