Social gliding motility in Myxococcus xanthus depends on the presence of Type IV pili. To begin to examine the role of pili in social motility, 17 mutants were identified which had lost social motility, but still expressed pili. Four of these mutants carry point mutations which mapped to a locus upstream of the recently identified pilS, pilR, and pilA genes. Sequencing of this locus revealed a gene with homology to pilT from Pseudomonas Aeruginosa. Sequencing of the four point mutations revealed that they occurred within the M. xanthus pilT locus. A markerless deletion within M. xanthus pilT, similar to the four point mutations, disrupted social gliding behaviour but did not interfere with pilus formation or pilus-dependent cell-cell agglutination. Using time-lapse videomicroscopy, residual social motility was observed in dsp- strains (known to be deficient in fibril but not pilus production); this was not observed in a delta pilT dep- double mutant. Two genes flanking pilT were also sequenced, and found to have homology to pilB and pilC from P. aeruginosa. Markerless deletions within these genes caused both pilus and social-motility defects. These results indicate that M. xanthus pilB and pilC are required for pilus biogenesis, while pilT is required for assembled pili to play their role in social motility. Thus, pilB, pilT, pilC, pilS, pilR and pilA form a contiguous cluster of pil genes required for social motility.