Dsp mutants of Myxococcus xanthus have a complex phenotype with abnormal cell cohesion, social motility, and development. All three defects are the result of a single mutation in the dsp locus, a region of DNA about 14 kilobases long. Cohesion appears to play a central role in social motility, since nonsocial mutants exhibit weak agglutination or, in the case of Dsp cells, no agglutination (L. J. Shimkets, J. Bacteriol. 166:837-841, 1986). However, Dsp cells can be agglutinated by cohesive strains of M. xanthus. This provided the opportunity to examine the role of cohesion during development by comparing the developmental phenotype of Dsp cells with that of Dsp cells mixed with cohesive strains. Dsp mutants were unable to complete any of the developmental behaviors: aggregation, fruiting body formation, developmental autolysis, and sporulation. Contact with cohesive strains seemed to restore some developmental characteristics to the Dsp cells. When allowed to develop with wild-type cells, Dsp cells accumulated in fruiting bodies and underwent developmental autolysis, but did not form a significant portion of the spore population. Igl mutants, which may be similar to the previously described frizzy mutants, are cohesive strains that are unable to form fruiting bodies. Mixing Igl cells with Dsp cells under developmental conditions resulted in fruiting body formation, although the Dsp cells were unable to form significant levels of myxospores. In spite of their inability to sporulate under developmental conditions, Dsp mutants did not appear to be defective in the sporulation process. In fact, they formed normal levels of myxospores in response to the chemical inducer glycerol.