Myxococcus xanthus has two nearly independent genetic systems, A and S, which appear to mediate adventurous (single-cell) movement and social (group) movement, respectively. In addition to a notable reduction in group movement, social motility mutants exhibit decreased biofilm formation, cell cohesion, dye binding, fibril production, and fruiting body formation. The stk-1907 allele, containing transposon Tn5 insertion omega DK1907, was introduced into wild-type cells and many social motility mutants. This allele, which was epistatic to most social motility mutations, caused wild-type and most mutant cells to exhibit increased group movement, cell cohesion, dye binding, and production of cell surface fibrils. The presence of the stk-1907 allele in dsp mutants, which almost completely lack cell surface fibrils, did not result in these phenotypic changes; therefore, stk-1907 is hypostatic to dsp mutations. Those mutants which exhibited increased group movement and cell cohesion with the stk-1907 allele also had increased fruiting body formation, but no significant changes in spore production were observed. These results suggest that fibrils may mediate cell cohesion, dye binding, and group movement. Additionally, the results suggest that the dsp locus contains genes involved in subunit synthesis, transport, and/or assembly of fibrils. The wild-type and mutant alleles of stk were cloned and studied in merodiploids. The mutant allele is recessive, suggesting that Tn5 omega DK1907 caused a null mutation in a gene which acts as a negative regulator of fibril synthesis. The stk-1907 allele appears to cause utilization of the A motility system for group movement, possibly because of increased fibril production.