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Phylogenetic relationships of the species belonging to the genus Myxococcus were elucidated based on the sequences of 16S rRNA genes and 16S-23S rRNA gene internal transcribed spacer (ITS) regions. The Myxococcus species were consequently classified into four distinct groups. The type strain of Myxococcus coralloides occupied an independent position (Group 1); it has been recently reclassified as Corallococcus coralloides. Group 2 comprised the type strains of both Myxococcus virescens and Myxococcus xanthus, and some strains assigned to Myxococcus flavescens. The type strain of M. flavescens was contained in Group 3 along with the strains of Myxococcus fulvus. Group 4 included the strains belonging to C. coralloides, M. fulvus, and M. stipitatus. The type strain of M. fulvus that was allocated outside Group 4 in the 16S rRNA gene tree belonged to Group 3 in the ITS tree. These results strongly suggest that the morphological characteristics of Myxococcus species are not consistent with the phylogenetic relationships. The Myxococcus species must therefore be redefined according to the phylogenetic relationships revealed in this study.

In an attempt to isolate indigenous marine myxobacteria from coastal samples, we obtained two swarm forming bacteria. Both isolates formed cell aggregates which, at least in one isolate, developed to fruiting body-like structures consisting of a mass of myxospore-like cells. The optimum NaCl concentrations for their growth were between 2 and 3%, comparable to the NaCl concentration of seawater. This growth characteristic strongly suggests that the two isolates are specific marine bacteria. The 16S rDNA sequence studies indicated that the two isolates were related to the genus Nannocystis. Based on the phylogenetic distances between branches, we concluded that the isolates should be assigned to two new myxobacterial genera.