A general MS-based screen for unusually hydrophobic cellular small molecule-RNA conjugates revealed geranylated RNA in Escherichia coli, Enterobacter aerogenes, Pseudomonas aeruginosa and Salmonella enterica var. Typhimurium. The geranyl group is conjugated to the sulfur atom in two 5-methylaminomethyl-2-thiouridine nucleotides. These geranylated nucleotides occur in the first anticodon position of tRNA(Glu)(UUC), tRNA(Lys)(UUU) and tRNA(Gln)(UUG) at a frequency of up to 6.7% (~400 geranylated nucleotides per cell). RNA geranylation can be increased or abolished by mutation or deletion of the selU (ybbB) gene in E. coli, and purified SelU protein in the presence of geranyl pyrophosphate and tRNA can produce geranylated tRNA. The presence or absence of the geranyl group in tRNA(Glu)(UUC), tRNA(Lys)(UUU) and tRNA(Gln)(UUG) affects codon bias and frameshifting during translation. These RNAs represent the first reported examples of oligoisoprenylated cellular nucleic acids.