The database on small ribosomal subunit RNA structure contained 1804 nucleotide sequences on April 23, 1993. This number comprises 365 eukaryotlc, 65 archaeal, 1260 bacterial, 30 plastldial, and 84 mttochondrial sequences. These are stored in the form of an alignment in order to facilitate the use of the database as input for comparative studies on higherorder structure and for reconstruction of phylogenetic trees. The elements of the postulated secondary structure for each molecule are Indicated by special symbols. The database is available on-line directly from the authors by ftp and can also be obtained from the EMBL nucleotide sequence library by electronic mall, ftp, and on CD ROM disk.