Nonsense-mediated decay (NMD), also called mRNA surveillance, is an evolutionarily conserved pathway that degrades mRNAs that prematurely terminate translation. To date, the pathway in mammalian cells has been shown to depend on the presence of a cis-acting destabilizing element that usually consists of an exon–exon junction generated by the process of pre-mRNA splicing. Whether or not mRNAs that derive from naturally intronless genes, that is, mRNAs not formed by the process of splicing, are also subject to NMD has yet to be investigated. The possibility of NMD is certainly reasonable considering that mRNAs of Saccharomyces cerevisiae are subject to NMD even though most derive from naturally intronless genes. In fact, mRNAs of S. cerevisiae generally harbor a loosely defined splicing-independent destabilizing element that has been proposed to function in NMD analogously to the spliced exon–exon junction of mammalian mRNAs. Here, we demonstrate that nonsense codons introduced into naturally intronless genes encoding mouse heat shock protein 70 or human histone H4 fail to elicit NMD. Failure is most likely because each mRNA lacks a cis-acting destabilizing element, because insertion of a spliceable intron a sufficient distance downstream of a nonsense codon within either gene is sufficient to elicit NMD.