We have isolated a yeast conditional mutant which rapidly ceases synthesis of mRNA when subjected to the nonpermissive temperature. This mutant (rpbl-1) was constructed by replacing the wild-type chromosomal copy of the gene encoding the largest subunit of RNA polymerase II with one mutagenized in vitro. The rapid cessation of mRNA synthesis in vivo and the lack of RNA polymerase II activity in crude extracts indicate that the mutant possesses a functionally defective, rather than an assembly-defective, RNA polymerase II. The shutdown in mRNA synthesis in the rpbl-1 mutant has pleiotropic effects on the synthesis of other RNAs and on the heat shock response. This mutant provides direct evidence that the RPB1 protein has a functional role in mRNA synthesis.