The Ribosomal RNA Operon Copy Number Database (rrndb) is an Internet-accessible database that provides annotated information on the copy number of rRNA operons in prokaryotes. The database addresses the lack of readily available data on individual rRNA gene sequences and copy numbers, which are crucial for microbial identification and evolution studies. The rrndb includes detailed information linked to external websites such as the Ribosomal Database Project, GenBank, PubMed, and culture collections. It contains over 250 entries with data from full-genome sequencing projects, and it is accessible via a web interface that allows users to sort and search for information. The database is managed using MySQL and Java Server Pages, and it is updated quarterly with new information. The rrndb is valuable for researchers studying microbial ecology and evolution using 16S rRNA gene sequences.The Ribosomal RNA Operon Copy Number Database (rrndb) is an Internet-accessible database that provides annotated information on the copy number of rRNA operons in prokaryotes. The database addresses the lack of readily available data on individual rRNA gene sequences and copy numbers, which are crucial for microbial identification and evolution studies. The rrndb includes detailed information linked to external websites such as the Ribosomal Database Project, GenBank, PubMed, and culture collections. It contains over 250 entries with data from full-genome sequencing projects, and it is accessible via a web interface that allows users to sort and search for information. The database is managed using MySQL and Java Server Pages, and it is updated quarterly with new information. The rrndb is valuable for researchers studying microbial ecology and evolution using 16S rRNA gene sequences.