An insertion of transposon Tn5-lac, omega 4519, generates a lacZ fusion with a Myxococcus xanthus promoter expressed during both vegetative growth and development. Sequence analysis of the junction of omega 4519 with M. xanthus DNA shows that the insertion is in frzF, a homologue of cheR from Salmonella typhimurium. When frzF- (or frzCD-) cells are starved for nutrients at modest densities, they aggregate to form a radial pattern and produce fewer than 1% of the wild-type complement of spores. At higher densities, frzF::omega 4519 cells form 'frizzy' aggregates and produce 80-90% of the wild-type complement of spores. In contrast, when cells with both a frzF- (or frzCD-) and an sglA1 mutation are allowed to develop at either low or high cell densities, they produce frizzy aggregates containing a near wild-type complement of heat-resistant spores. In addition to suppressing the density dependence of fruiting-body morphogenesis, the sglA1 mutation also suppresses the sporulation defect caused by two different frzF- mutations and a frzCD- mutation. In contrast, a mutation in a different S motility gene, sglG1, does not suppress the frz- mutations. Thus, the suppression of frz- mutations by sgl- mutations is allele-specific, and depends on the sgl allele, but not the frz allele. Because the phenotypes of frz- mutations have been determined in a (suppressing) sglA1 genetic background, the frz genes may play more central roles in development than initially recognized.