Premature termination of translation has often been associated with decreased mRNA accumulation in plants, but the affected step in gene expression has not been identified. To investigate this problem, the expression of wild-type and mutant alleles of the bean phytohemagglutinin (PHA) gene has been examined in tobacco cells and transgenic plants. Measurement of mRNA decay rates in stably transformed cell lines demonstrated that premature nonsense codons markedly destabilized the mRNA. This decreased stability was also reflected by decreased accumulation of transcripts containing premature nonsense codons in transgenic plants. The positional dependence of the nonsense codon effect was evaluated by introducing premature nonsense codons at different distances from the PHA AUG start codon. Transcripts with nonsense codons about 20, 40 or 60% of the way through the normal PHA coding region yielded highly unstable mRNAs, whereas a transcript with a nonsense codon at 80% was as stable as wild-type. The ability to recognize and rapidly degrade certain transcripts with early nonsense codons could provide plant cells with a means to minimize the production of wasteful and possible deleterious truncated proteins.