Interest in finding small RNAs (sRNAs) in bacteria has significantly increased in recent years due to their regulatory functions. Development of high-throughput methods and more sophisticated computational algorithms has allowed rapid identification of sRNA candidates in different species. However, given their various sizes (50 to 500 nucleotides [nt]) and their potential genomic locations in the 5= and 3= untranslated regions as well as in intergenic regions, identification and validation of true sRNAs have been challenging. In addition, the evolution of bacterial sRNAs across different species continues to be puzzling, given that they can exert similar functions with various sequences and structures. In this study, we analyzed the enrichment patterns of sRNAs in 13 well-annotated bacterial species using existing transcriptome and experimental data. All intergenic regions were analyzed by WU-BLAST to examine conservation levels relative to species within or outside their genus. In total, more than 900 validated bacterial sRNAs and 23,000 intergenic regions were analyzed. The results indicate that sRNAs are enriched in intergenic regions, which are longer and more conserved than the average intergenic regions in the corresponding bacterial genome. We also found that sRNA-coding regions have different conservation levels relative to their flanking regions. This work provides a way to analyze how noncoding RNAs are distributed in bacterial genomes and also shows conserved features of intergenic regions that encode sRNAs. These results also provide insight into the functions of regions surrounding sRNAs and into optimization of RNA search algorithms. R ecently, small noncoding RNAs (sRNAs) have been under closer scrutiny as mediators and regulators of gene expression (1-5). This class of RNAs has been found to play a variety of roles in important cell functions (6, 7). Typically composed of 50 to 500 nucleotides, sRNAs are known to control plasmid replication, bacterial virulence, and various stress responses (8-11).An interesting aspect of sRNAs is the wide diversity of their functional mechanisms. sRNAs can repress or stimulate gene expression posttranscriptionally by pairing their targets through base complementarity; a target can be, but is not limited to, an mRNA or a protein. sRNAs that regulate other RNAs can be cis encoded or trans encoded. A cis-encoded sRNA is typically encoded adjacent to its regulatory target on the same strand as a riboswitch or on the opposite strand to an antisense sRNA. In most cases, they will base pair to their targets or change the secondary structure to inhibit ribosome binding (12)(13)(14). In contrast, a trans-encoded sRNA is encoded away from its target, has a lower base complementarity to its target, and can potentially bind multiple targets (15).With advances in high-throughput sequencing technologies (16), it is now possible to sequence gigabases of nucleotides in a matter of hours (17). Aided by sRNA prediction algorithms, these large data sets are paving the way for cont...