The ecmA and ecmB genes of Dictyostelium are expressed in prestalk and stalk cells. They encode components of the slime sheath, the extracellular matrix that surrounds the migrating slug, and the stalk tube, the matrix that encases stalk cells. We have generated, by homologous gene disruption, a mutant in which the ecmB gene is inactivated but the strain develops normally. In contrast, ecmA null mutant strains develop to form abnormally long and thin standing slugs. While the slime sheath of mutant slugs appears to be normal in electron microscopic observations, the sheath material remaining on the substratum after the slug travels through it is abnormally susceptible to breakage. After a short period of migration the axial ratio of mutant slugs decreases to that of normal slugs and, at culmination, normal fruiting bodies are produced. These data suggest that the EcmA protein has its primary role during slug formation, where it contributes to the strength of the slime sheath, and that the function of the EcmB protein is dispensible.