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Escherichia coli F-18, a normal human fecal isolate, is an excellent colonizer of the streptomycin-treated mouse large intestine. E. coli F-18 Col-, a derivative of E. coli F-18 which no longer makes the E. coli F-18 colicin, colonizes the large intestine as well as E. coli F-18 when fed to mice alone but is eliminated when fed together with E. coli F-18. Random sequences of E. coli F-18 DNA were cloned into pRLB2, a par-B-stabilized derivative of pHC79. The entire gene library was transformed into E. coli F-18 Col- and fed to streptomycin-treated mice. The mouse large intestine selected a predominant clone which contained a recombinant plasmid (pRLB7) that enhanced E. coli F-18 Col- colonizing ability 100-fold but did not stimulate colicin synthesis. Moreover, pRLB7 simultaneously improved the survival of E. coli F-18 Col- in stationary phase in vitro, utilizing nutrients derived from mouse cecal mucus, and stimulated synthesis of both type 1 fimbriae and three E. coli F-18 Col- outer membrane proteins (74, 71, and 69 kDa). The 6.5-kb E. coli F-18 DNA sequence in pRLB7 does not contain either the fim operon or pilG (hns), both known to be involved in type 1 fimbrial synthesis. The sequence encodes six proteins, all smaller than the three E. coli F-18 Col- outer membrane proteins whose synthesis it stimulates. Collectively, the results suggest that the cloned E. coli F-18 DNA sequence contains one or more regulators of E. coli F-18 Col- operons expressed in the mouse large intestine in vivo and in isolated mouse cecal mucus in vitro.