An agglutination assay was used to study cell cohesion in the myxobacterium Myxococcus xanthus. Vegetative cells agglutinated in the presence of the divalent cations Mg2+ and Ca2+. Agglutination was blocked by energy poisons that inhibit electron transport, uncouple oxidative phosphorylation, or inhibit the membrane-bound ATPase. However, energy was not required for the maintenance of cells in the multicellular aggregate. Cyanide, a strong inhibitor of agglutination, did not cause cells to dissociate from the aggregate even when shear forces were applied. While gliding motility was not necessary for agglutination, some gliding mutants exhibited aberrant agglutination that was generally correlated with cell behavior. Cells with an intact social motility system were cohesive and glided in large multicellular swarms. Cells with a mutation in their social motility system were 5- to 10-fold less cohesive and tended to glide as single cells. One group of social motility mutants, known as Dsp, did not agglutinate.