The Ribosomal Database Project (RDP) is a curated database that provides ribosome-related data, analysis services, and associated computer programs. It includes phylogenetically ordered alignments of ribosomal RNA (rRNA) sequences, derived phylogenetic trees, rRNA secondary structure diagrams, and various software for handling, analyzing, and displaying alignments and trees. The data are available via anonymous ftp, electronic mail, and gopher. The email server provides ribosomal probe checking, approximate phylogenetic placement of user-submitted sequences, screening for chimeric nature of newly sequenced rRNAs, and automated alignment. The RDP provides data, programs, and services related to the ribosome. It includes ribosomal RNA sequences from major sequence repositories and direct submissions. These sequences are organized and presented in aligned and phylogenetically ordered form. Each sequence is annotated with its organismal source, cellular compartment, origin of sequence data, and other relevant information. The RDP also examines original publications and updates annotations, strain designations, and organism names. Submitters and/or the public sequence databases are notified of possible errors. The small subunit (SSU) rRNA alignments currently comprise sequences from approximately 120 Archaea, 2100 Bacteria, and 440 Eucarya. A representative alignment of 76 prokaryotic SSU rRNA sequences is also available. The number of large subunit (LSU) rRNA sequences remains at 150. A phylogenetic tree is available for the sequences in the posted prokaryotic SSU rRNA alignment. It has been assembled from appropriately overlapping subtrees, each of which has been inferred using maximum-likelihood analysis. The RDP also offers a collection of SSU and LSU rRNA secondary structure diagrams in PostScript format generated and supplied by R. Gutell and his collaborators. The RDP provides a variety of programs, including DNArates and two data and graphics conversion programs. The RDP is largely supported by the National Science Foundation, Biological Instrumentation and Resources Division. Research assisted by any RDP service should cite the RDP, the release number, and this article. Future changes and additions include unaligned rRNA sequences being made available for some analyses, a full interface to the RDP on the World Wide Web, and new programs and services.The Ribosomal Database Project (RDP) is a curated database that provides ribosome-related data, analysis services, and associated computer programs. It includes phylogenetically ordered alignments of ribosomal RNA (rRNA) sequences, derived phylogenetic trees, rRNA secondary structure diagrams, and various software for handling, analyzing, and displaying alignments and trees. The data are available via anonymous ftp, electronic mail, and gopher. The email server provides ribosomal probe checking, approximate phylogenetic placement of user-submitted sequences, screening for chimeric nature of newly sequenced rRNAs, and automated alignment. The RDP provides data, programs, and services related to the ribosome. It includes ribosomal RNA sequences from major sequence repositories and direct submissions. These sequences are organized and presented in aligned and phylogenetically ordered form. Each sequence is annotated with its organismal source, cellular compartment, origin of sequence data, and other relevant information. The RDP also examines original publications and updates annotations, strain designations, and organism names. Submitters and/or the public sequence databases are notified of possible errors. The small subunit (SSU) rRNA alignments currently comprise sequences from approximately 120 Archaea, 2100 Bacteria, and 440 Eucarya. A representative alignment of 76 prokaryotic SSU rRNA sequences is also available. The number of large subunit (LSU) rRNA sequences remains at 150. A phylogenetic tree is available for the sequences in the posted prokaryotic SSU rRNA alignment. It has been assembled from appropriately overlapping subtrees, each of which has been inferred using maximum-likelihood analysis. The RDP also offers a collection of SSU and LSU rRNA secondary structure diagrams in PostScript format generated and supplied by R. Gutell and his collaborators. The RDP provides a variety of programs, including DNArates and two data and graphics conversion programs. The RDP is largely supported by the National Science Foundation, Biological Instrumentation and Resources Division. Research assisted by any RDP service should cite the RDP, the release number, and this article. Future changes and additions include unaligned rRNA sequences being made available for some analyses, a full interface to the RDP on the World Wide Web, and new programs and services.