Strain S2 is a lecithin (or phosphatidylcholine)- solubilizing bacterium, which was isolated from the rice rhizosphere in rural areas of Beijing, China. On the basis of a polyphasic study involving phenotypic tests, physiological and biochemical tests, 16S rDNA sequence analysis, G+C content determination and DNA-DNA hybridizations analysis, strain S2 was identified as Pseudomonas alcaligenes. P. alcaligenes S2 was mutagenized with Tn5 and four mutants showing decreased or increased solubilizing ability of lecithin were isolated based on the halo size around colonies on the solid plate supplemented with egg yolk. To characterize the genes of P. alcaligenes S2 involved in solubilization of lecithin, the EcoR I fragments of the chromosomes from the four mutant strains carrying a single transposon were cloned, and the DNA sequences flanking the Tn5 were determined. The Tn5 insertion sites in the mutants M808, M1329 and M1400, showing decreased solubilizing ability of lecithin, were found to be located in the xcpS, xcpX and xcpW , respectively, whose products XcpS, XcpX and XcpW were the components of type II secretion pathway. Complementation of xcpS, xcpX and xcpW could restore the corresponding mutants M808, M1329 and M1400 to solubilize lecithin. The data suggested that mutation in one of these xcp genes would lead to the absence of mature lecithinase secretion into the extracellular medium. The data also indicated that the secretion of lecithin-hydrolyzing enzyme of P. alcaligenes was via type II secretion pathway. In the mutant M20 showing increasing lecithin-hydrolyzing activity, the interrupted gene showed 86% identity with chpA of Pseudomonas aeruginosa PAO1, whose product plays an important role in controlling twitching motility of the bacterial cells.