Many bacteria regulate the expression of specialized gene sets in response to the population density at high density. This regulatory mechanism is called autoinduction or quorum-sensing. One autoinducer-deficient mutantYQ1 was obtained by screening Mesorhizobium huakuii mutated with mariner transposon. The autoinducer synthase gene was confirmed by sequencing and named mrhI. The internal sequence was cloned into lacZ transcriptional fusion plasmid pVIK112 to obtain the PmrhI-lacZ fusion strainYQ3, from which we studied the expression and regulation of autoinducer synthase gene mrhI of M. huakuii. The result showed that the mrhI gene can be expressed only in the presence of added AHLs and was regulated in a cell density-dependent way. All of these results indicated that M.huakuii AS9 has the quorum sensing system, and in the same time, they also give some clues to the further study of the quorum sensing system of rhizobia species.