Myxococcus fulvus HW-1, a salt-tolerant bacterial strain, which was isolated from a coastal environment, changes its behavior with different salinities. To study the relationship between behavioral shifts and the adaption to oceanic conditions, the HW-1 strain was randomly mutagenized using transposon insertion, producing a dispersed-growing mutant, designated YLH0401. The mutant did not develop fruiting bodies and myxospores, was deficient in S-motility, produced less extracellular matrix and was less salt tolerant. The YLH0401 strain was determined to be mutated by a single insertion in a large gene of unknown function (7011 bp in size), which is located in a horizontally transferred DNA fragment. The gene is expressed during the vegetative growth stage, as well as highly and stably expressed during the development stage. This horizontally transferred gene may allow Myxococcus to adapt to oceanic conditions.