Current in vitro assays for RNA editing in kinetoplastids directly examine the products generated by incubation of pre-mRNA substrate with guide RNA (gRNA) and mitochondrial (mt) extract. RNA editing substrates that are modeled on hammerhead ribozymes were designed with catalytic cores that contained or lacked additional uridylates (Us). They proved to be sensitive reporters of editing activity when used for in vitro assays. A deletion editing substrate that is based on A6 pre-mRNA had no ribozyme activity, but its incubation with gRNA and mt extract resulted in its deletion editing and production of a catalytically active ribozyme. Hammerhead ribozymes are thus sensitive tools to assay in vitro RNA editing.