Xanthomonas campestris NK-01 produces amounts of xanthan polysaccharide. Nonmucoid mutants, defective in synthesis of xanthan polysaccharide, were isolate after treatment with ethylmethanesulfonate. To isolate genes essential for xanthan polysaccharide synthesis complete libraries of DNA fragments from wild-type Xanthomonas campestris NK-01 were constructed in E. coli HB101. The pooled clone bank was conjugated en masse from E. coli into a nonmucoid mutant. Kanamycin-resistant exconjugants were then screened for the ability to form mucoid colonies. Analysis plasmids form muciod exconjugants indicated that overlapping segments of DNA had been cloned. These plasmids were tested for complementation of nine additional nonmucoid mutants. A 13. 4 kb region of DNA was determined to contain at least some genes essential for xanthan polysacchdride synthesis.