Plasmids pUR291 and pNZ521. containing lacZ gene, maturation protein and proteinase P genes, were transferred into X. campestris either by conjugation or by transformation Plasmid pNZ521 was also conjugally transferred into X. campestris XMT1 a transformant carrying plasmid pUR291. Ah the constructed strains were evaluated for xanthan gum production in either a medium of 50% whey or the same medium supplemented with 1.5% lactose or 1.5% glucose. Mixed cultures either with transconjugants or with transformants were tested for xanthan gum production as well.