Studies with yeast DNA topoisomerase mutants indicate that neither topoisomerase I nor II appears to be essential for transcription by RNA polymerase II. However, plasmids carrying transcriptionally active genes are found to be extremely negatively supercoiled when isolated from mutants lacking topoisomerase I. Supercoiling occurs during transcriptional elongation rather than during transcriptional activation. It takes place in the absence of topoisomerase I and does not seem to be dependent on topoisomerase II since it can occur at the nonpermissive temperature in a top1-top2 ts mutant. Whether this change in linking number is due to an unusual form of topoisomerase II or whether it is due to a new enzyme has yet to be determined. The results suggest that topoisomerase I is normally required to relax transcriptionally induced supercoils. A model is discussed which considers the role of topoisomerases in the movement of RNA polymerase along the DNA template.