Escherichia coli K-12 harboring a part of the structural genes for the Klebsiella K2 capsular polysaccharide (cpsK*) expresses a large amount of K2 capsular polysaccharide as a thick capsule in the presence of plasmids carrying rmpA and rcsB. We have previously shown that expression of the Klebsiella K2 capsule in E. coli HB101 harboring cpsK* depends on the presence of rmpA, a regulatory gene from a large plasmid of Klebsiella pneumoniae Chedid (O1:K2). E. coli K-12 JM109, however, produces only a small amount of K2 capsular polysaccharide, even in the presence of plasmids carrying rmpA as well as the cpsK* structural genes. Introduction of the rcsB gene, a positive regulator of colanic acid capsule synthesis in E. coli K-12 which was cloned from HB101 on a plasmid, into JM109 cells carrying cpsK* and rmpA, results in the expression of a thick K2 capsule. By Northern (RNA) hybridization analysis, rcsB has been found to enhance transcription of a long strand of mRNA (longer than 14 kb) from cpsK*. These E. coli transformants which produce a thick K2 capsule also express colanic acid production at high levels. Therefore, rcsB can act as a positive regulator of Klebsiella K2 capsule production and two capsular polysaccharides can be expressed in E. coli simultaneously. With a somewhat different strain background, we have found that both of the colanic acid regulators, rcsA and rcsB, contribute to the basal level of Klebsiella K2 capsule expression but that the presence of multicopy rcsB in either an rcsB or an rcsA mutant of E. coli is sufficient to increase the expression of K2 capsular polysaccharide. These results suggest further parallels between the regulation of colanic acid synthesis in E. coli and the regulation of Klebsiella K2 capsule synthesis.