Sphingomonas elodea ATCC 31461 produces gellan gum and Sphingomonas sp. ATCC 53159 produces diutan gum as capsules, firmly bound to the bacterial cells. Capsular polysaccharides produced by Sphingomonas strains (called sphingans) have related structures and therefore genes involved in sphingan synthesis are similar but not identical. Three genes controlling attachment of gellan and diutan to the bacterial cell were identified by individual or combined inactivation of involved genes followed by phenotypic evaluation of resulting slime mutants to demonstrate slime characteristics. Chromosomal gene deletions constructed to inactivate genes dpsM and dpsN of the diutan biosynthetic gene cluster and gelM-gelN and gelN of the gellan biosynthetic gene cluster were shown to result in production of polysaccharide that was not attached to the cell surface. Insertional inactivation of another gene gelI from the gellan biosynthetic gene cluster also resulted in the slime phenotype, demonstrating its role in capsule formation. Inactivation of the homologous gene dpsI should also result in synthesis of diutan in the slime form. The deletion and insertion mutants were evaluated for slime forming characteristics using microscopic evaluation, cell pellet formation, and cell clumping. Cells from slime mutants grew in suspension, gave cell pellet upon centrifugation and were free and motile under the microscope, whereas cells from capsular strains grew in clumps, were difficult to pellet and remained in gum matrix in microscopic evaluation.