We have previously reported the isolation and initial characterization of a mutation in Saccharomyces cerevisiae, designated mod5‐1, that reduces the capacity of altered tyrosine tRNAs to suppress ochre nonsense mutations. The mutation results in the virtual elimination of the modified tRNA nucleoside, N‐Δ2‐(isopentenyl) adenosine, normally found adjacent to the anticodons of certain tRNA species. We demonstrate here that MOD5 codes for Δ2‐isopentenylpyrophosphate: tRNA‐Δ2‐isopentenyl transferase, or a protein that regulates its synthesis.