Decay rates of individual mRNAs in the yeast Saccharomyces cerevisiae can vary by 10- to 20-fold. To determine the basis for the rapid degradation of the mRNA encoded by the yeast MFA2 gene we have used a genetic screen to isolate mutations that increase the stability of this transcript. Analysis of point mutations obtained from this screen, and of additional lesions constructed in vitro, indicated that the MFA2 3'-untranslated region (UTR) contains sequences that specify rapid mRNA decay. Moreover, the lesions that affected mRNA decay rate also affected the process of mRNA deadenylation. Mutations in one region of the 3' UTR both decreased the rate of poly(A) shortening and increased the stability of an intermediate form in the decay pathway with an oligo(A) tail (approximately 10 nucleotides). Mutations in a second region primarily increased the stability of the oligo(A) form. These results suggest that the decay of the MFA2 mRNA initiates with the shortening of the poly(A) tail and there are specific sequences within the 3' UTR that stimulate poly(A) tail shortening as well as subsequent steps in the decay pathway. Given the similarity of this decay pathway to that seen for some mammalian mRNAs, these results suggest that mRNA deadenylation may be a common mechanism of mRNA turnover.