Comparison of 16S ribosomal ribonucleic acid (rRNA) sequences has emerged as a powerful tool for bacterial phylogeny. However, earlier studies often only included one or a few species per genus, and it is not sure whether the rRNA sequences could discriminate closely related species. The genus Serratia is composed of ten species, some being up to 60% related by DNA hybridization. The reverse transcriptase/primer extension method was used to determine 1,492 to 1,509 nucleotides in each of ten Serratia 16S rRNA sequences. All rRNA sequences determined were unique. The phylogenetic tree obtained with the neighbour-joining method showed a cluster of Serratia species distinct from both Escherichia coli and Proteus vulgaris. S. fonticola--whose position in the genus Serratia is questioned--was clearly included in the Serratia branch and grouped within the psychrophilic Serratia species. Variable regions in the Serratia rRNA molecules were identified and could serve as the basis for a specific probe design.