Abstract: The nonsense-mediated mRNA decay (NMD) pathway, present in most eukaryotic cells, is a specialized pathway that leads to the recognition and rapid degradation of mRNAs with premature termination codons and, importantly, some wild-type mRNAs. Earlier studies demonstrated that aberrant mRNAs with artificially extended 3'-untranslated regions (3'-UTRs) are degraded by NMD. However, the extent to which wild-type mRNAs with long 3'-UTRs are degraded by NMD is not known. We used a global approach to identify wild-type mRNAs in Saccharomyces cerevisiae that have longer than expected 3'-UTRs, and of these mRNAs tested, 91% were degraded by NMD. We demonstrate for the first time that replacement of the natural, long 3'-UTR from wild-type PGA1 mRNA, which encodes a protein that is important for cell wall biosynthesis, with a short 3'-UTR renders it immune to NMD. The natural PGA1 3'-UTR is sufficient to target a NMD insensitive mRNA for decay by the NMD pathway. Finally, we show that nmd mutants are sensitive to Calcofluor White, which suggests that the regulation of PGA1 and other cell wall biosynthesis proteins by NMD is physiologically significant.