This laboratory previously showed that an RNA derived from 5S ribosomal RNA could be used as a carrier to harbor a nucleic acid "tag" for monitoring genetically engineered or naturally occurring bacteria. The prototype system expressed a specific tagged RNA that was stable and accumulated to high levels. For such a system to be useful there should, however, be little limitation on the sequence composition and length of the insert. To test these limitations, a collection of insertion sequences were created and introduced into the artificial 5S rRNA cassette. This library consisted of random 13- and 50-base oligonucleotides that were inserted into the carrier RNA. We report here that essentially all of the insert-containing RNAs are stable and accumulate to detectable levels. Tagged RNAs were produced by both plasmid-borne and chromosomally integrated expression systems in E. coli and several Pseudomonas strains without obvious effect on the host cell. It is anticipated that in addition to its intended use in environmental monitoring, this system can be used for in vivo selection of useful artificial RNAs. Because the carrier lends stability to the RNAs, the system may also be useful in RNA production.