Subcellular localization signals for several mRNAs are positioned in their 3′ untranslated regions (UTR). We have utilized the human α- and β-actin 3′ UTRs as signals for colocalizing hammerhead ribozymes with a lacZ target mRNA. Ribozyme and target genes containing matched or unmatched 3′ UTRs were cotransfected into 12-day-old chicken embryonic myoblast and fibroblast (CEMF) cultures and assayed by in situ hybridization (ISH) using a dual label, antibody sandwich procedure, and dual fluorescence microscopy to monitor intracellular colocalization. β-galactosidase localization in transfectants was visualized by incubation with X-gal and also quantitated by an o-nitrophenyl β-D-galactopyranoside (ONPG) assay. We found that the percentage of colocalization using the matched α- or β-actin 3′ UTR (α–α or β–β) was enhanced approximately threefold relative to unmatched 3′ UTRs. The increase in ribozyme-mediated inhibition of β-galactosidase activity observed when matched 3′ UTRs were used was consistent with the observed percentage of colocalization. These results represent the first direct demonstration that mRNA localization signals (zipcodes) can be utilized to enhance intracellular ribozyme efficacy.