This study is part of a project where the Chlamydomonas reindhardtii rbcL 5’UTR is analyzed for its effect on transcript stability. Two different mutated rbcL 5’UTRs were fused to GUS reporter genes, and introduced into the Chlamydomonas chloroplast genome. In vivo transcript accumulation was determined by northern blot assay.One of the 5’UTR modifications was an addition of 9 nucleotides to the 5’terminus of the transcript and a deletion of the nucleotide at the original start of transcription. The other mutation changed the nucleotide sequence in a 5’UTR stem-loop without changing its secondary structure. It was found that both modifications rendered the transcripts unstable, and significantly reduced accumulation of GUS transcripts.