Ensifer adhaerens is a predatory Gram negative bacterium which, unlike Bdellovibrio , is not an obligate predator. This organism specifically attacks the common soil bacterium, Micrococcus luteus . When Ensifer encounters Micrococcus , both in natural conditions and in the laboratory, the predator spreads along populations of the prey organism, in a behavior termed "tracking." Micrococcus is then rapidly destroyed by Ensifer .We hope to create mutants of Ensifer defective in tracking behavior by identifying strains that lack exopolysaccharide or that do not display swimming motility.To that end, we have introduced four different types of transposons into Ensifer with good efficiency, and screened several thousand colonies for non-mucoid and non-swimming phenotypes.We have identified one mutant defective in swimming motility; it is currently being evaluated for its ability to "track" when exposed to Micrococcus .Finally, we succeeded, by PCR and cloning, in obtaining a small segment of the 16s rRNA gene of Ensifer .Phylogenetic analysis of this DNA segment shows strong relatedness to the fast growing rhizobia, another class of common soil bacteria. Ensifer is thus another predatory prokaryote, which may play a role in bacterial population structure in the soil.